Stromatolite Fossils in the Hakatai Shale
A Day Hike from Phantom Ranch - Grand Canyon National Park
Location: Near Phantom Ranch / Grand Canyon National Park
Geological Formations: Vishnu Group, Zoroaster Plutonic Complex, Bass Limestone, Hakatai Shale, Shinumo Quartzite, Tapeats Sandstone, Bright Angel Shale
Other Attractions: Outcrops of stromatolite fossils in the Hakatai Shale and dramatic 360° views from the top of the mesa
Hiking Difficulty: A steep route requiring some scrambling and hand and foot boulder climbing to reach the 'Utah Flats' vicinity, a dry and exposed mesa top in wild use area AP9. Suitable for day hikes or longer by experienced Grand Canyon backcountry hikers.
Access: Via the Bright Angel or the South or North Kaibab Trails. The route to the mesa begins at Bright Angel Campground Camp Site #1. Phantom Ranch or Bright Angel Campground provide convenient base camps for this hike.
USGS 7.5 Minute Arizona Topo Map: Phantom Ranch
During May and June 2000 I did a series of hikes in Grand Canyon National Park. The Grandest of these hikes and rockhound adventures, several years in the planning and preparation, occurred over 7 days from May 26th through June 1st on a 50 mile or so solo backcountry excursion along a route I dubbed the 'Grand Angel Traverse', which eventually brought me to Bright Angel Campground and Phantom Ranch at the bottom of the Grand Canyon.
There I rested and relaxed in backpacker's decadence after enduring the rigors and challenges of 110°+ heat in the shade (of which there was almost none) and nearly non-existent sources of water for four days while I backpacked down the Grandview Trail to and below Horseshoe Mesa, and then westbound along the Tonto Platform, contouring a dendritic, fractal path around Cottonwood Canyon, Grapevine Canyon, Boulder Creek Canyon, Lonetree Canyon, and Cremation Creek Canyon via the Tonto Trail from Cottonwood Creek to the South Kaibab Trail.
During my R&R at Phantom Ranch, a serendipitous last minute change of my backcountry itinerary plus a good dose of just plain dumb luck led me to a remarkable outcrop of billion year old Precambrian fossils in a sandstone member of a Proterozoic Grand Canyon Supergroup Formation known as the Hakatai Shale.
Above left is a north looking overview from the South Rim showing the location of the mesa just west of Phantom Ranch where I found stromatolite fossils exposed in the Hakatai Shale. Phantom Ranch lies below and a day hike away from the outcrop and is hidden in this view at the bottom of Bright Angel Canyon, near the inner gorge of the Colorado River. Above right is a telephoto close-up of the mesa area, with an arrow pointing to the general vicinity of the stromatolite bearing outcrop I found in the Hakatai. This mesa is bounded on the north by Phantom Creek Canyon, on the east by Bright Angel Canyon, on the south by the inner gorge of the Colorado River, and on the west by a canyon separating it from Cheops Pyramid, to which it is connected by a saddle on its northwest side.
The exposures of Paleozoic sedimentary formations forming the upper regions of Grand Canyon are unrivaled in both completeness and beauty anywhere else on Earth, and along the Grand Angel Traverse I enjoyed closely inspecting, photographing and hiking through of some of the finest and most aesthetic formations within Grand Canyon. The Tonto Platform, which saddles the inner gorge of the Colorado River within the Grand Canyon, rests upon the Tapeats Sandstone, the lower most and oldest of 14 distinct Paleozoic formations revealed by magnificent erosion through geologic column at Grand Canyon. These 14 formations record and reveal conditions and life existing throughout nearly all of the Paleozoic Era, from the beginning of the Cambrian Period through the middle Permian, a span of time covering 270 million years of evolution prior to the appearance of the dinosaurs at the beginning of the Mesozoic Era about 250 million years ago.
Above left is a gorgeous outcrop on the west side of Cottonwood Creek, to the best of my knowledge unnamed. It is composed from the top down by the Redwall Formation, the Temple Butte Formation, the Muav Limestone and the Bright Angel Shale. The overall lithologic sequence of these formations provides chronological evidence of an ancient sea that invaded from the present day west, the sandstone of the Tapeats recording higher energy, near shore conditions, overlain by the mudstones and shales of the Bright Angel recording a deeper and quieter shelf environment of deposition, which are in turn overlain by the limestones and dolomites of the Muav, recording even deeper and quieter conditions of deposition further offshore. A disconformity representing over 100 million years occurs between the Muav and the Temple Butte, where records from the Ordovician and Silurian Periods of the Paleozoic Era are missing from the geologic column at Grand Canyon.
Although I applied in January for an early May backcountry permit for this particular hike, when my permit finally arrived in the mail the Park Service had awarded me dates in late May. Due to the later than optimal permit and unseasonably hot temperatures, I had to carry 3 days worth of water as I backpacked across my route, which is a significant amount when you've got to deal with scorching temperatures and the shadeless environment of the Tonto Platform. The Tonto Trail led right up to the brink of the inner canyon of the Colorado River at several points along my route, bringing me within just a quarter mile of all the water I could ever want. The only problem was it was a quarter of a mile straight down...
It was getting on in the afternoon of the fourth day of my Grand Angel traverse when I reached the intersection of the South Kaibab and Tonto Trails. There I left the Tonto Platform and my westerly route across early Cambrian strata in the Bright Angel Shale atop the Tapeats Sandstone to follow the South Kaibab Trail north and down a seemingly ever steepening corkscrew of switchbacks through exposures of even more ancient rocks of Proterozoic time, consisting of several lower Grand Canyon Supergroup formations and the Vishnu Group.
The Grand Canyon is dissected by a number of eroded faults and the South Kaibab Trail terminates at the erosional bottom of the canyon spanning Bright Angel Fault. Shown above left is the confluence of Bright Angel Creek and the Colorado River. Bright Angel Creek is a significant perennial Grand Canyon tributary of the Colorado which flows down from the north side through the canyon it has opportunistically carved through broken and shattered rock along the Bright Angel Fault. The abundant and accessible water and riparian shade was a most welcome sight as I approached the refreshing end of my descent, crossing to the north side of the Colorado River by way of Grand Canyon's Black Bridge to Bright Angel Campground and Phantom Ranch.
There is a ranger station, emergency medical facilities and heliport at Bright Angel Campground / Phantom Ranch, as well as a food and lodging concession which is operated in no small part for the benefit and service of the ever popular, daily mule trains whose participants make their way down one day on muleback from the South Rim via the Bright Angel Trail for an overnight stay at Phantom Ranch, and then back out to the South Rim on their mules via the South Kaibab Trail the following day. However, backpackers can also make reservations for meals and lodging at Phantom Ranch, where backcountry permits are not required (assuming single day, no campsite hikes from and to either Rim getting there and out).
The present resort at Phantom Ranch was constructed in 1921 by the Fred Harvey Company. A preceding tourist camp known as Rust's Camp had been operated here by David Rust, who put in a cable tramway across the Colorado River in 1907. President Roosevelt stayed here in 1913 resulting in a change of name to Roosevelt Camp. Mary Jane Colter, the architect of many other structures within Grand Canyon National Park, designed buildings here and is credited with naming Phantom Ranch.
I claimed one of the last available campsites in Bright Angel Campground where I had backcountry permit reservations for the night and hurriedly dropped my pack and freshened up as best I could with a quick dunk in Bright Angel Creek. Then I beat it up the trail to Phantom Ranch, where I arrived just in time for the first of several meals I had strategically reserved months in advance. That steak dinner along with all the ice water and tea I could drink was one of the finest and most appreciated feasts in my recollection, and the luxury of cooled air inside the dining hall at Phantom Ranch was nothing short of heavenly!
Above left is a view of the interior of the dining hall, which also serves as a cantina, lounge, library, game room and general hang out and cool place with free ice water when it is not filled with diners for breakfast and dinner, both of which were served in two shifts. This proved to be a great place to meet and compare experiences and swap stories with other hikers who were passing through and staying at nearby Bright Angel Campground or Phantom Ranch. Yes, those are Tecate beers on the table, and they even sold packs of Marlboros at the concession here. I assume the cigarettes were for the benefit of the mule riders and not the hikers. After the cantina shut down for the evening I meandered back to Bright Angel Campground, cooled down and sated with food and water, to a well earned rest atop still baking hot bedrock.
Pictured above right is a nicely shaded outdoor lounging and meeting area at Phantom Ranch with several of the cabins which can be rented in the background. While staying at Phantom Ranch I attended several group talks and lectures which were presented here and at an adjacent meeting area by Ranger Naturalist Pam Cox. During her informative and entertaining presentations Ranger Pam covered several topics including Grand Canyon fossils, flora, nocturnal life and a recounting of the historic first navigation and survey of the Colorado River through Grand Canyon by Major Powell's 1869 expedition.
My backcountry permit itinerary and Phantom Ranch meal reservations were planned around two nights at Bright Angel Campground with an intervening overnight hike back out onto the Tonto Platform to camp at large along the Clear Creek Trail on the north side of the Colorado River. I had planned to take off after a Phantom Ranch breakfast the next morning, but learned then that on-the-spot reservations were available for dorms and more meals due to a greater than normal number of cancellations which were attributed to the unseasonably hot weather. In spite of being dead tired, I had spent a less than thoroughly restful night atop my sleeping bag in Bright Angel Campground due to the heat. And as mindblowingly awesome as the rocks were out there, it only took a short meditation to realize other recreational and educational opportunities were presenting themselves in lieu of venturing back out on the shadeless, waterless Tonto Platform for a couple of more days.
So I signed up for a bunk in an air conditioned Phantom Ranch dorm for the next two nights. And while I get by okay on oatmeal, noodles, dehydrated vegetables and a custom blended trail mix I call "Pack Rat's Delight No. 3", the food at Phantom Ranch was scoring high points with me. So I also signed up for several more of their deluxe home cooked style meals and sack lunches to fill in where I had planned on 'eating out' along the Clear Creek Trail. :)
I spent the remainder of the morning and most of the afternoon casually exploring the area around Phantom Ranch and hanging out along the Colorado River. Because it is released from the bottom of Glen Canyon Dam upstream, the river water is so cold it actually hurts at first to stick your feet in. I soon discovered that even though I was sitting on a rock exposed to the sun I could readily modulate my body temperature between hot and cold just by adjusting the depth of my feet. After several hours I finally managed to work my way up to a full body dunk in the Colorado, but in spite of the 110°+ air temperature I could only stand this for a few seconds. Before Glen Canyon Dam was built, it used to be a not unheard of practice for hard core Grand Canyon backcountry hikers to cross the Colorado at strategic points during favorable conditions by stripping down and swimming across it with their gear floated on an inflatable mattress. Attempting such crossings would be suicidal now with hypothermia onsetting very rapidly.
In the vicinity of Phantom Ranch the inner gorge carved by the Colorado River cuts into and exposes the supracrustal Vishnu Group, often referred to as the "crystalline core". These are the most ancient rocks exposed at Grand Canyon and consist of micaceous and quartzose schists and gneisses, amphibolites, and hornblende schists which were metamorphosed and folded about 1.75 billion years ago at the roots of a long lost mountain range. These metamorphic rocks are generally dark to nearly black in color and are intruded with great plutons, dikes and sills of granites, granitic pegmatites, aplites and granodiorites. These intrusives are generally known as the Zoroaster Plutonic Complex, which was emplaced before, during and after the Vishnu metamorphism over a span of several hundred million years. Spectacular dikes and sills of beautiful pink granite intrude the Vishnu in the vicinity of Phantom Ranch. An inbound mule train negotiating the trail cutting across the granite dike pictured above left provides but an inkling of its scale.
A number of rafts and several dories floated by while I was hanging out at the river, and some landed at a beach near the Black Bridge. I gleaned from conversations with other hikers observing them that some did not hold the rafters in such high regard and they referred to their vessels as "pig boats". Pictured above right is a motorized, galaxy class pig motherboat that landed at Phantom Ranch while I was doing R&R checking out nearby alluvial deposits of incredibly diverse river rocks. I have met Canyoneers who are strong and experienced hikers but who also relate appreciating the float trips they have taken through Grand Canyon. While I am convinced that hiking the Grand Canyon backcountry is the ultimate way to see and experience it (backpacking is in fact the only way to venture into the vast majority of it), the main thing that holds me off from a trying a float trip is their astronomical cost. Unfortunately, non-commercial, private permits to float the Grand Canyon are essentially unobtainium due to the decade(s) long "waiting list" for them. However, touring by and hiking from a commercial raft loaded with geologists, mineralogists, paleontologists, sedimentologists, and other academics I could annoy with incessant questions regarding what we were seeing might not be such a bad deal...
Later in the afternoon I sought out the cool of the Phantom Ranch cantina and I was checking out their library of Grand Canyon books when Ranger Pam came in and announced she was about to give a talk on Grand Canyon fossils, which of course I attended. Afterwards I told Ranger Pam I wanted to do a day hike into the Grand Canyon Supergroup and inquired as to whether any of the trails north of the river near Phantom Ranch would be suitable. She responded with a description of a route which could be ascended through the Vishnu and then the overlying Bass Limestone and Hakatai Shale enroute to the top an adjacent mesa to the west, a vicinity known as Utah Flats.
The Bass Limestone and Hakatai Shale are very ancient geologic formations in Grand Canyon National Park, the lowermost members of the Unkar Group, which together with the Nankoweap Formation, the Chuar Group, and the Sixtymile Formation, make up the Proterozoic Grand Canyon Supergroup, a several mile thick layer of sedimentary and volcanic formations which was laid down over the schist, gneiss and granite of the Vishnu Group over a period extending from 1250 to 700 million years ago. After deposition, the Supergroup formations were subsequently subjected to faulting, tilting and uplift. The Supergroup was completely eroded away and lost from above the Vishnu Group in all but isolated grabens, which are downfaulted blocks produced by crustal stretching. Unlike the heavily metamorphosed Vishnu Group, the sedimentary and igneous natures of surviving Grand Canyon Supergroup members are generally well preserved in discontinuous and scattered outcrops throughout the Grand Canyon.
Various Unkar Group members are exposed in seven vicinities, from east to west: the "Big Bend" region in eastern Grand Canyon; Clear Creek; Bright Angel Creek; Phantom Creek/Phantom Ranch; Crystal Creek; Shinumo Creek/Hakatai Canyon; and Bedrock Creek/Tapeats Creek. Of these, the exposures in the Bright Angel Creek and Phantom Creek/Phantom Ranch vicinities are the most accessible and least demanding to reach by hiking.
A Day Hike from Phantom Ranch
After a refreshing night's sleep in a cool dorm followed by a hearty Phantom Ranch breakfast downed with luxurious amounts of hot black coffee, by 7 a.m. the next morning I was once again ready to get away from it all for a while and began my ascent to visit some Supergroup rocks using the route up through the Vishnu described by Ranger Pam. The steep inner canyon walls cut through the hard, crystalline Vishnu by the Colorado River and its tributaries within Grand Canyon are formidable and generally unnegotiable barriers to all but birds. A typical Vishnu wall is depicted above left. However here and there the rock has been shattered by faulting and worn back by erosion, presenting opportunities of varying difficulty for those of us lacking wings to scramble and climb through breaks in the otherwise impenetrable barriers.
In addition to the Canyon spanning Bright Angel Fault, the lesser Tipoff, Cremation and Phantom Faults are among those shaping the nearby terrain and playing roles in the preservation and exposure of Supergroup formations in the general vicinity of Phantom Ranch. Leading out of Bright Angel Campground Campsite 1 adjacent to the campground bridge over Bright Angel Creek is the beginning of the route, depicted above right and on the map below, that I followed up through the Vishnu that morning. Yellow diamonds indicate the bottom and top of the route from Bright Angel Campground to the edge of the mesa.
Above left is an overview of Phantom Ranch, and above right is Bright Angel Campground, both pictures taken from a vantage point several hundred feet up the route, the beginning of which is indicated on the Bright Angel Campground shot with a yellow diamond. While they use native species, these large beautiful shade trees are cultivated by the Park Service and even protected from beavers with wire guards.
Routes are more scrambly, bushwhacky, generally less defined and harder to negotiate and follow than trails. There are routes and then there are routes, just as there are trails and then there are trails. There are many more routes than trails within Grand Canyon and some are tough indeed. A legendary Canyoneer named Harvey Buchart hid under a cloak of respectability as a University of Northern Arizona mathematics professor in nearby Flagstaff when he wasn't off hiking or climbing in the Grand Canyon at nearly every available opportunity. Harvey meticulously logged and recorded details regarding an amazing 10,000 miles plus of Grand Canyon backcountry hiking, a good deal of which was done along routes he discovered and pioneered. Among Harvey's backcountry accomplishments are numerous first ascents made during more than 50 years of hiking and climbing the most remote, unexplored and difficult routes through the Grand Canyon backcountry.
While little more than a stroll in the park for the likes of Harvey Buchart I'm sure, negotiating this route to and from the mesa top required the better part of a day for me. It is rather steep but I do like getting right down to business over short distances. I proceeded slowly and cautiously, carrying just a day pack with my lunch, camera gear and 4 liters of water, and did not experience too many frights or bad moments. There is slippery footing on loose talus in numerous places with some nasty to no doubt fatal exposures if you lose it, and hand and foot scrambling is required to get over some larger boulders near the route's top just before the (relatively) level and more easily negotiated top surface of the mesa is reached. But I think this route is within the capabilities of many others, assuming of course you are sufficiently motivated by cool rocks and are not overly mesmerized or paralyzed by heights.
Carrying a heavier pack up and down would increase the difficulty and risks significantly. At points the only way I knew I was still on the route was that I was still making headway going in the general direction of up. Coming down later that afternoon was more taxing on my knees and my nerves in spite of being more familiar on the return, but still manageable with caution reinforced by the fact that you don't have any other options by that point anyway... ;)
The Bass Limestone
The first Unkar Group formation encountered along the route is the Bass Limestone, the oldest Supergroup formation, and second in age only to the Vishnu Group. The Bass lies disconformably atop the Vishnu. The lithology of the Bass Limestone is predominately dolomite with lesser occurrences of arkose, sandy dolomite, shale and argillite. Breccias and conglomerates both underlie and occur throughout the Bass. Pictured above left and right are exposures of the Bass Limestone taken near a saddle in the route approximately half way up the ascent to the mesa, waypointed by my Garmin eTrex GPS receiver at approximately UTM 12S 0400981, 3996034, 3372 feet elevation.
The sedimentary structures, thickness trends and overall composition of the Bass Limestone suggest deposition of its lower members by an easterly transgressing sea and of its upper members during a regressive phase as the sea retreated towards the west. During the sea's maximum incursion carbonates and deep water mudstones accumulated in western Grand Canyon, while in the eastern regions shallow water mudstones were deposited. Evidence of a regression in upper members includes ripple marks, mud cracks and deposits of oxidized shales. The Bass generally erodes to form cliffs or terraces of stair-stepped cliffs, with the harder dolomites forming the risers and the argillites and shales forming steep treads.
Sedimentary structures to be found in exposures of the Bass Limestone include ripple marks, desiccation cracks, intraformational breccias and conglomerates, and graded beds which are associated with stromatolite fossils. Especially noteworthy within the Bass Limestone are characteristic occurrences of stromatolite beds, the most ancient fossils at Grand Canyon.
The Significance of Stromatolites
Stromatolites are the oldest known fossils recording life on Earth, dating back more than 3 billion years. Stromatolite fossils are micro-laminated sedimentary structures, the by-products of life processes of colonial organisms known as cyanobacteria and other microbes. Cyanobacteria derive their name from the pigments phycocyanin and allophycocyanin, which give many of them a characteristic blue green color. Cyanobacteria are photosynthetic prokaryotes, which are primitive organisms making their living with cellular chemistries similar to plants, but lacking the DNA-packaging cellular nucleus characterizing all eukaryotes - single and multicellular plants and animals. Stromatolites thrived in the early seas, with stromatolite populating species building reefs resembling those built by modern day corals.
The image of the stromatolite fossil cross section at right was provided courtesy the Museum of Paleontology at the University of California, Berkeley. Note the progressive layering of dome shaped features atop one another in this specimen, a characteristic structure created by reef building stromatolite colonies.
Stomatolites were initially considered to be blue-green algae, but they actually pre-date the oldest fossil evidence of algae and apparently all other eukaryotes by approximately 2 billion years, with the oldest stromatolites dating to the Early Archaean Eon. During the Proterozoic Eon, stromatolites were widespread and abundant in Earth's oceans. By the close of the Proterozoic, pressure from more highly evolved life had markedly decreased stromatolite populations.
Photosynthesizing cyanobacteria were responsible for the creation of Earth's oxygen atmosphere during the Archaean and Proterozoic Eras, and in doing so these relatively primitive organisms played a tremendously important role in shaping the course of succeeding evolution and ecological change throughout Earth's history. The oceans and atmosphere had a very different chemistry lacking free oxygen, unsuitable for life as we know it today, prior to the appearance and activity of cyanobacteria. They were the dominant lifeform on Earth for over 2 billion years and the builders of Earth's first reefs.
The primitive cyanobacteria lived in huge masses with populations in the billions per square meter that could form floating mats or extensive reefs. The reef like layers were produced as calcium carbonate precipitated over the growing mat of bacterial filaments. Photosynthesis in the cyanobacteria depleted carbon dioxide in the surrounding water, initiating the precipitation.
The precipitated calcium carbonate particles, along with fine grains of sediment carried by the sea water, were trapped within the sticky layer of mucilage surrounding the bacterial colonies, which continued to grow upwards through the sediment to form a new layer. The mineralized layers of stromatolites were accreted through repeated iterations of this process. These layered deposits, which have a distinctive "signature" are called laminar stromatolites.
Most often, stromatolites appear as variously-sized arches, spheres, or domes. In some cases, the stromatolites were infiltrated with a mineral-rich solution fossilizing the bacteria along with the byproduct layers of precipitated calcium carbonate and trapped sediments, but most commonly only the byproduct layers are preserved.
Stromatolites survive to this day but they are nearly extinct, living a precarious existence in only a few localities worldwide, one at Shark Bay (Hamelin Pool Marine Nature Reserve) in Australia and another at several islands in the Bahamas. The image of still living Shark Bay stromatolites at right was provided courtesy Discover West Holidays. The Shark Bay stromatolites are found around the edge of Hamelin Pool, a nearly land-locked embayment where one section of coast is lined for over 80km with stromatolites of various shapes and sizes. Measurements have revealed a growth rate of less than half a millimeter annually for the fastest growing Shark Bay stromatolites, dating some of the still growing structures there at over one thousand years old.
Precambrian stromatolites grew to massive sizes, but geologically younger stromatolites tend to be cropped by more evolved grazing organisms, and only build into large structures in areas where grazers are excluded. Stromatolites are able to survive in Australia's Hamelin Pool because its water is twice as saline as normal sea water and many other forms of competitive and predatory marine life cannot survive there.
After a preliminary scan with a telephoto lens, I was very tempted to venture over to and explore some rather appealing stair stepped outcrops of the Bass Limestone just off the route leading to the mesa. This was the first time I had ventured into really accessible Supergroup exposures and I knew of the Bass's reputation for harboring stromatolite fossils. I very much wanted to find and photograph some stromatolites and these outcrops in the Bass looked pretty promising. However, it was still early in the day and I was also chomping at the bit to continue my ascent and attempt to reach exposures and take some documenting and scenic photos of the outrageously colorful Hakatai Shale above the Bass. So I decided to postpone venturing off the route and exploring the Bass for stromatolites until the return leg of my day hike when I would be more tired and in need of a break.
It soon became apparent to me that "Bass Limestone" was really somewhat of a misnomer due to the variety of rock types within this formation. After a first hand inspection I appreciate why "Bass Formation" has been proposed by some technical workers as a more accurate description. In fact, my experience has been that virtually all of the strata in Grand Canyon popularly designated and labeled as "Sandstones", "Limestones" and "Shales" exhibit complex structures and members composed of differing lithologic types.
Some grade smoothly into each other, creating a continuous record of deposition and geological and environmental conditions. Between others there are minor to major temporal gaps, nonconformities where depositional process were interrupted by a change in geologic conditions that produced no net deposition or erosion instead.
The geologic record provided by the Bass Limestone and the overlying Hakatai Shale is thought to be continuous and without any significant temporal gaps, with the "contact" between these two formations conformable, being the most gradational in eastern outcrops and sharpest in western outcrops. An underlying layer of conglomerate known as the Hotauta Conglomerate Member deposited in low areas of the eroded and hilly Vishnu terrain composes the lowermost unit of the Bass Limestone. A switchbacky section of the route up a wall above the stair stepped outcrops in the Bass traversed a number of intraformational conglomerate layers, one of which is shown above with my hat to provide some scale for the size of its clasts.
Above are views detailing another conglomerate layer towards the upper portion of the Bass with a 55 mm lens cap providing scale. In this occurrence the conglomerate clasts were not well sorted and tended to be more on the broken and angular side than well rounded, indicating they had not been transported very far from their source by the erosional processes depositing them here. I encountered intraformational conglomerate bearing strata in a number of places in the upper Bass along the route, but did not see enough of the formation to get a feel as to whether these conglomerate layers generally occurred as lens type deposits or if they were more or less continuous throughout the area.
Further up in the Bass I encountered outcrops alternating with a relatively high frequency between a hard, sandy dolostone and softer strata, some of which was also clast bearing. About this time a group of about a dozen turkey vultures appeared and circled over me. I knew they circled out of curiosity at my activity, but couldn't help myself from also wondering what they knew that I didn't... After about twenty minutes or so I guess they either concluded that I hadn't quite been broiled done by the sun yet, or maybe they just found a better thermal and drifted on about their business as I approached the upper end of the route to the edge of the mesa.
The Grand Canyon is of course bird heaven on Earth, with over 300 hundred species to be observed and over 130 known to nest in its diverse habitats and ecological niches created by extreme changes in elevation over short distances and the Colorado River. In contrast to the thermal riding turkey vultures I also observed white-throated swifts and violet-green swallows enthusiastically carving up the air off the mesa edges. Or heard the swifts and swallows would be a more accurate description most of the time, as when they are really getting in on and pass nearby they literally part the air with a startling whistle and are nowhere to be seen by the time you react and look for them. During this day hike I also encountered a number of ravens which are large enough to work thermals and soar but who are also no slouches in the cliffside aerobatics department, as well as several species of scrub dwelling birds atop the mesa I was not knowledgeable enough to identify.
The Hakatai Shale
I finally reached the edge of the mesa at the end of a vertical ascent along the route of about 1300 feet, approximately one quarter the total depth of the Grand Canyon. The final leg of the route climbs through a constricted erosion channel leading off the east side of the mesa that is clogged with large boulders. Hand and foot scrambling and climbing through these boulders is the only way to get through but most hikers confident in their balance and footing will not find this section overly difficult provided they are not loaded down with heavy, unwieldy packs. The mesa top is tilted up towards the south and west, with the highest point occurring at approximately 4120 feet above sea level on its southern edge, another 300 to 400 feet or so further above the route's egress on the eastern rim.
The mesa surface is extensively eroded in several areas, with curious and dramatically shaped outcrops of the Hakatai Shale exposed in these drainages, some of which bore a marked resemblance to stacks of pancakes. (I guess I had worked off breakfast by this time.)
Note the tilt that is characteristic of Grand Canyon Supergroup formations revealed in the Hakatai photo at left, where the stalk of a blooming century plant at the end of its life cycle forms the indicator of a natural 'tilt meter'.
The Hakatai Shale was deposited atop the Bass Limestone around the end of the Middle Proterozoic approximately one billion years ago. The regressing sea which deposited the upper members of the Bass Limestone produced deltaic conditions as it retreated, and their predominance marks the beginning of the Hakatai Shale.
The Hakatai consists of two lower members which are composed of mudstones and shales that erode to form gentle to moderate slopes, and an upper member composed of cliff forming beds of medium-grained quartz sandstone. The Hakatai is generally regarded to be the most colorful formation in the Grand Canyon, varying from purple to red to brilliant orange on outcrop due to oxidation of iron bearing minerals contained in its rocks.
Fossilized mud cracks, ripple marks and cross bedded sedimentary structures provide evidence the beds of the Hakatai were deposited in marginal marine environments, the mudstones and shales recording a low energy mud flat environment and the overlying sandstones recording a shallower, higher energy environment.
Because of the interesting outcrops created and revealed within the erosion channel entered at the egress of the route, I decided to follow it up and to the west and at times along a course roughly contouring along the south side of it during my initial exploration of the mesa and the Hakatai.
The sedimentary nature of the Hakatai is pronounced and clearly revealed by high frequency and sometimes paper thin laminations of shales in its lower members that alternate in color from a somewhat nondescript, light brown color to outrageous reddish purples. At right is a photo illustrating a characteristic structure. The variegation was caused by and records changing conditions along the margins of the ancient sea as it cyclically transgressed and regressed in sub-cycles following a generally regressive trend during the deposition of the Hakatai as it retreated towards the west.
As I hiked up the erosion channel in a generally westerly direction I also zigzagged across and explored nearby shelves and outcrops of the Hakatai exposed along the southern flank of the channel. Shown above is a close up of fossilized, cross bedded ripple marks I found in an eroding sandstone member of the Hakatai, waypointed by my eTrex at UTM 12S 0400588, 3996208, elevation 3829 feet. A 55 mm lens cap provides scale. These ripple structures and the relatively coarse sand clasts suggest these sediments were deposited by a higher energy environment than the finely stratified mudstone and shale members. The overall lithologic sequence from the underlying Bass Limestone, overlain by mudstone and shale and finally sandstone members of the Hakatai, indicate a chronological progression of increasingly shallower and higher energy environments of deposition.
By the time I found these fossilized ripple marks the heat was well over 100° and becoming bothersome even in the shade, which was in very short supply atop the mesa with the sun overhead, and my water was redlined with half of the 4 liters I'd brought along already consumed. It was a brutally clear and cloudless day that was just going to keep getting hotter and my need for water increasing. A little voice inside me was starting to run on about it being time to turn back.
While I was hiking around and inspecting the outcrop of ripple marked sandstone for some nicely sculpted ripples at an advantageous angle to the sun for a photo, I noticed several tan colored, lump shaped rocks weathering out in one area. A closer inspection of one of these revealed a curious shape that reminded me of a clam, but I knew the Hakatai dated from the mid Proterozoic... and then my eye found an entire clump of this material that bore a striking resemblance to a coral formation. Suddenly it dawned upon me that I had just found an outcrop of billion year old stromatolite fossils weathering out of the Hakatai Shale! Of course my excitement over this serendipitous discovery displaced that little voice nagging at me about heading back and I proceeded to spend the next hour or so surveying the immediate area for other outcrops of stromatolites.
I did not find extensive beds of stromatolite fossils exposed in the general vicinity of my search, indicated with a red X on the topo map above. However it was most engaging and rewarding to me to find several larger outcrops weathering out of the Hakatai that I thought quite fine. The outcrop of fossils pictured above preserves some nice three dimensional detail of the structures created by these Proterozoic stromatolites. A 55 mm lens cap provides scale.
The image above provides an overhead perspective of detail from an outcrop. A 55 mm lens cap provides scale. In the course of reading about Grand Canyon geology and preparing for my hikes I'd encountered a number of references to stromatolite fossils occurring in the Bass Limestone, but no mention of them in the Hakatai. So I had not anticipated the possibility of finding these stromatolites and needless to say I was fortunate to have been so lucky as to basically just wander across them during the least planned, researched and most spontaneous leg of my Grand Angel Traverse. Finding these stromatolite fossils was partly by chance, but there is another part that has to do with "being there". The best tip I know to pass on regarding finding cool rocks in the Grand Canyon backcountry is to be there so you can have the dumb luck.
This stromatolite "biscuit" had weathered completely out and separated from one of the outcrops. I think "Evolution" would be an appropriate title for the above picture of a human hand holding a stromatolite fossil created by cyanobacteria four or five orbits of our solar system about the galactic center or so ago. Needless to say the time I was able to spend visiting and documenting the fossilized remains left by these primordial ancestors was a highpoint with me.
I think there is a very good possibility there are other outcrops of stromatolites exposed in the Hakatai atop this mesa. While I did not recognize any others during the remainder of my exploration, that was really more of a hit and miss affair along the south side of the erosion channel that I followed westward across the mesa. I didn't have time to do anything approaching a systematic search and I could have easily missed spotting other outcrops along the south side of the channel I as I hiked through it. The channel cuts through and exposes what appeared to be more of the stromatolite bearing strata at the equivalent contour along its opposite side to the north, which was entirely unvisited by me.
There are also several other erosion channels cutting through and draining the mesa top in different directions besides the one I followed that I did not venture into. I'd like to do a more extended visit to the backcountry atop this mesa and spend several days exploring it for stromatolite fossils and photographing its rocks as well as the spectacular panoramic views of the Canyon afforded from the top of the mesa.
The Shinumo Quartzite
Along the southern and western edges of the mesa are exposures of the Shinumo Quartzite, a overlying Unkar Group formation that disconformably contacts the Hakatai Shale. Deposition of the Hakatai is believed to have been ended by tectonic activity producing a series of northwest trending, high angle reverse faults that was followed by a period of erosion prior to the deposition of Shinumo sands and sediments. After hiking up and meandering across the mesa in a generally westerly direction I reached cliffs along its western edge composed of erosion resistant quartzites and sandstones of the Shinumo.
The photo at left depicts a view to the northwest from the Shinumo Quartzite on the west side of the mesa, overlooking a side canyon separating the mesa and Cheops Pyramid in the background. This exposure of the Shinumo forms a ribbon of vertical cliffs beginning along the south and west sides of the mesa, around this side canyon, surrounding Cheops Pyramid on its east, south and west sides and then on along the south side of Isis Temple where it finally pinches out between the Hakatai and the Bright Angel Shale.
Below the Shinumo Quartzite at the base of Cheops Pyramid are spectacular orange and reddish purple outcrops of Hakatai Shale. Making up Cheops Pyramid above the Shinumo are the Paleozoic Bright Angel Shale, Muav Limestone and Temple Butte Formation, concealed for the most part beneath a good deal of talus, capped by a steep walled prominence composed of Redwall Formation. The normally ubiquitous Tapeats Sandstone is missing from below the Bright Angel Shale where hard, resistant quartzites and sandstones of the Shinumo eroded to a hilly topography with highs poking through and pinching out the much later deposited and subsequently eroded Paleozoic Tapeats.
A disconformity representing a gap in geologic time of roughly half a billion years exists where the Shinumo contacts overlying Paleozoic formations. In most places at Grand Canyon there are no remnants of the Middle to Late Proterozoic Grand Canyon Supergroup intervening between the Early Proterozoic Vishnu Group and the overlying Paleozoic formations. Where the Paleozoic column directly overlies the Vishnu Group, an even more tremendous gap spanning over a billion years, known as the Great Unconformity, is missing from the Precambrian geologic record at Grand Canyon.
The images above left and right were photographed from the Shinumo cliffs on the southwest edge of the mesa, and record the spectacular exposures the Bass Limestone, Hakatai Shale and Shinumo Quartzite across a side canyon at the base of Cheops Pyramid, named after the pharaoh who built the Great Pyramid in ancient Egypt. In the image above left the Bass Limestone tops the folded crystalline core of the Vishnu as a ribbon of pinkish, vertical cliffs. Atop the Bass are purplish, softer shale and mudstone members of the Hakatai Shale which have eroded to form receding, stair stepped mounds. Above right, further towards the north and further up the base of Cheops Pyramid are brilliant orange outcrops of the Hakatai, which are striking and highly visible features viewed by millions annually from miles away on the South Rim in the general vicinity of Grand Canyon Village and nearby outlooks. Atop the Hakatai lies a purplish band of vertical cliffs formed by the Shinumo Quartzite.
I continued following the cliffs on around the southwest edge of the mesa, eventually doubling back towards the east. After an hour or so of following the cliffs, a good deal of which was spent just sitting or standing along the edge and taking in the spectacular views, I looped back around in a kind of figure 6 maneuver that took me over the crown of the mesa. Most of the surface atop the mesa that is not carved down by erosion is covered with a thin, rocky soil atop a somewhat horseshoe shaped cap of Tapeats Sandstone and Bright Angel Shale disconformably overlying the tilted Unkar Group members. This soil is sufficient to support cacti and scrub vegetation typical of that occurring in the Bright Angel Shale atop the Tonto Platform. The scrub was frequently so dense and thick as to demand some attention and respect if I did not want to do duty as a pincushion in boots.
Apparently mule deer find this scrub appetizing, probably during wetter and cooler seasons,
as I did not encounter any fresh signs of deer during my jaunt about the mesa. Or perhaps the scrub is not so tasty and they just venture out here for the view too. Once dropping back down onto the southern flank of the erosion channel I had previously followed across the mesa, I found my way back to the stromatolites and pondered and enjoyed them for a few more moments before I headed back down the channel to the route's egress on the east side of the mesa, and back down the route to Phantom Ranch.
It was wearing on in the afternoon when I reached the saddle near the stair stepped outcrops of Bass Limestone I had resolved to search for stromatolite fossils on my way back. However, I had become pretty hot by this time, and my knees and leg muscles were weary and feeling a little rubbery from all the previous activity and now the steep descent, only about half way through at that point. I was feeling pretty satisfied with my hike about the mesa top, and as I wiped my face down with a moistened bandanna and swigged down the last of my water, it didn't take too much consideration for me to decide the rest of my day hike needed to be all down hill and exploring those outcrops in the Bass was going to wait for another visit.
After reaching Phantom Ranch I took a very refreshing shower in the dorm and changed into the last of my clean clothes. I spent a little while reorganizing and preparing my pack so that it would be ready and I wouldn't have to waste any cool morning minutes attending to such details prior to beginning my long hike out and up to the South Rim the next day.
The salad, cornbread and stew dinner I had that evening really hit the spot. I attended a lecture after dinner presented by Ranger Pam on nocturnal animals in Grand Canyon during which she talked extensively about bats and also got out a UV light to search for scorpions. I'm sorry to report no scorpions were spotted in the general vicinity of the lecture area, but that was probably just as well for the sake of the peace of mind of those hikers who would later be sleeping on the ground in Bright Angel Campground. After her lecture and Q&A was over I described to Ranger Pam what I had seen earlier that day and thanked her again for her description of the route to the mesa.
After dark I retired to the cantina and read from their library of Grand Canyon books while enjoying the cool air, a couple of coffees and hydrating up on ice water for my hike out beginning early the next morning. I also wrote and sent off several Phantom Ranch and Grand Canyon post cards, which get stamped "Mailed by Mule" and are carried out by pack mule, as is all the trash and garbage from Phantom Ranch operations. While I tend to generally prefer the areas of Grand Canyon with the lowest human densities, I did enjoy several spontaneous conversations I had with other hikers and the general ambiance of the cantina that evening. I loaded up on more coffee, water, and as much eggs, bacon, pancakes and fruit as I could stuff in at the early breakfast the next morning, and hit the trail well before 6 AM. On my way out of Phantom Ranch I detoured back towards the Black Bridge and spent some moments inspecting a nearby archeological site where there are Anasazi ruins. Then I doubled back and crossed the Colorado River via the Silver Bridge and followed the River Trail westbound along the river bottom until it turns south and begins ascending the Pipe Creek Canyon, whereupon it becomes the Bright Angel Trail.
Above are views of the Silver Bridge which spans the Colorado River about half a mile downstream of the Black Bridge. Note the beautiful pink Zoroaster granite intruding the Inner Canyon wall above it. The Silver Bridge was built to carry a six inch, high pressure water pipe across the Colorado River and of course this bridge can also be crossed on foot. This transcanyon pipeline is fed from Roaring Springs, miles away at the head of Bright Angel Creek on the north side of Grand Canyon. After crossing the river via this pipe suspended beneath the Silver Bridge, the Roaring Springs water is pumped up all the way up to the South Rim, where it is the sole source feeding the storage tanks supplying water to the millions of annual visitors and tourists using the services at Grand Canyon Village. In the picture above right, my hat provides some scale for the diameter of the pipe. It is covered with a layer of insulation and it was difficult to imagine just this pipe keeping up with the water requirements of the South Rim. There is flooring installed on the Black Bridge so that beasts of burden crossing it cannot look down through to the river below, but the Silver Bridge lacks such amenities.
At left is a view from the Bright Angel Trail taken near 3 Mile Rest House. This heavily mule beaten trail follows a long established Indian route exploiting erosion and breaks in the rocks along the Bright Angel Fault. The relatively straight line rather than meandering course of Bright Angel Canyon leading off in the distance to the horizon at the North Rim is due to erosion following an opportunistic course along the broken and shattered rock bordering the fault. Indian Garden lies several miles down the trail in the densely treed riparian area below, and further off in the distance marked with an arrow is the mesa where I found the stromatolite fossils in the Hakatai the previous day.
The hike out from Phantom Ranch to the South Rim via the Bright Angel Trail is steeper where it ascends through the erosion resistant Vishnu Group and the Tapeats Sandstone, and then the average grade mellows out for several miles while traversing a broad, gently sloping shelf of the softer Bright Angel Shale coming through the general vicinity of Indian Garden. The hike out gets relatively steep again when the more erosion resistant dolomitic limestones of the Redwall Formation are encountered below 3 Mile Rest House. The grade remains relatively steep for the remaining three miles to the South Rim as the trail ascends numerous switchbacks through the Supai Group, Hermit Formation, Coconino Sandstone, Toroweap Formation and the Kaibab Formation, all of which erode to form vertical to relatively steep cliffs.
I reached the Bright Angel trailhead on the South Rim, 4420 feet above the Colorado River and roughly 10 trail miles and a billion and a half years up the geologic column from Phantom Ranch around 4 PM that afternoon. While the Bright Angel Trail is the most heavily trafficked and physically least demanding trail in the Park connecting the South Rim to the Colorado River, I was in no particular hurry on my way out and stopped frequently for short rests with snacks and to enjoy the vistas stretching out before me and back into time. The baking heat and cloudless skies were of some concern, and the hike out would not have been as casual had it not been for readily available water along this corridor trail at Indian Garden and rest houses located about 3 and 1.5 miles away from the trailhead on the South Rim. I stopped at every opportunity to wet down my hat, head and shirt and replenish an extra couple of liters I carried for wetting down on the trail, and exploited the available water to travel light, stay (relatively) cool and hike throughout the afternoon.
The Next Time
The mesa adjacent to Phantom Ranch really beckoned to me for exploration and once up on it I didn't want to leave in spite of blistering heat, diminishing water and shadeless, exposed conditions. I think a lengthier stay of several days duration atop this mesa would make for a really rewarding and memorable Grand Canyon backcountry experience. Of course you'd have to obtain a backcountry permit to stay on the mesa overnight, and you'd have to pack in all your water for the duration. But packing a couple of day's worth of water up the route I followed would not be unthinkable during cooler seasons. Even if I return for "just" another day hike I will more be more generous with my water ration next time. I had not planned on staying out as long as I did on this day hike, and in hot weather especially more water is desirable to provide increased endurance and greater margin of safety.
I think bringing along a 25 foot or so length of rope might not be a bad idea to help safely raise and lower a heavier and bulkier gear and water laden pack over some of the boulders near the route's egress on the eastern edge of the mesa instead of on your back where its inertia might spoil your balance at the wrong moment. If you are more of a climber type and have a closer fitting, frameless pack you like for scrambling that can contain a few days worth of gear and water, that would be the one to bring along.
The use area for the mesa is AP9, Phantom Creek, classified by the Park Service as a wild zone, which is recommended only for "highly experienced Grand Canyon hikers with proven route-finding ability. Indistinct to non-existent routes require advanced route-finding ability. Water sources scarce to non-existent." The downside to hiking in wild zones alone is if you have an otherwise non-fatal accident that prevents you from getting out on your own you are likely to wish it had killed you. The upside is if you enjoy your solitude while in the backcountry it is unlikely to be interrupted. You shouldn't have too hard a time getting a backcountry permit for camping at large in AP9, but coupling your permit to also stay at Bright Angel Campground might complicate getting dates you want due to the heavy traffic and pressure for permits at Bright Angel Campground from hikers doing rim to river and back overnighters via the corridor trails from the South Rim.
One possible return scenario I'm contemplating would entail hiking down to Phantom Ranch from the South Rim via the South Kaibab Trail and staying at Bright Angel Campground or Phantom Ranch that night, returning up the route I followed the next morning onto the mesa to do a several day circumnavigation and exploration of it. Besides looking for more fossils, there is a saddle on the northwest side of the mesa that could be crossed over to visit Cheops Pyramid, and the northern edge of the mesa bounded by Phantom Creek Canyon should also prove interesting and scenic to explore.
I think the general plan follows up the backcountry hike about the mesa with a stayover puttering around at Phantom Ranch for a day or two of R&R there chowing down on Phantom Ranch grub, spending at least one night showered clean in a bed, fishing for rainbows on the Colorado River, checking out the bridges and Indian ruins, attending the ranger lectures, enjoying free entertainment by mule train riders whose butts are so sore it hurts them to sit down, swapping a few stories and lies with other hikers eating meals there or hanging out in the cantina, and maybe checking out Vishnu and Supergroup rocks on a day hike or overnighter into nearby Phantom Canyon. Or maybe an ascent to the North Rim via the North Kaibab Trail to take some snow on Grand Canyon pics on the long way back to the South Rim... ;)