Thursday, December 21, 2017

2017 Geology Posts and Photos That "Never Quite Made It"

Chile's "Towers of Blue"; Rounding the Horn at the End of the World; A "Warm" Remnant of the Patagonian Ice Sheet; A Sure Sign of New England Spring; Iceland's "Golden" Waterfall; Hrafnabjörg - A Classic Table Mountain; Hutton's Section and "Deep Time"; Eroded "Bridal Path" Dike of the Franconia Range; A Sentinel Butte with an Uplifting Story to Tell; Roundup on the Coconino Plateau; Trekking the Geology of the Tonto Platform.   

By the time the end of the year rolls around, there are always a few posts that were never written. And so, with this final post of the year – in what has become a tradition on my blog for six years running – here’s my end-of-the year post of some of the stuff that "never quite made it." Please visit the same for 2012 (here), 2013 (here), 2014 (here), 2015 (here) and 2016 (here). Coordinates have been provided that will take you there when pasted into an on-line mapping program such as Google Earth.

Chile's "Towers of Blue"
Torres del Paine National Park, Chile
Go to the summit of Cerro Paine Grande: 50°59′56″ S, 73°05′43″ W 

View of Cordillera del Paine and Valle Frances from Lake Pehoé Facing North

Cordillera del Paine is the majestic centerpiece of Chile's Torres del Paine National Park, established in 1959 and anointed a UNSECO Biosphere Reserve in 1978. Pronounced PIE-nay, which means "blue" in the language of the native Tehuelche, it's an eastern subordinate range of the Southern or Patagonian Andes. The Andes is the southern component of the American Cordillera, the mountainous backbone of North and South America, which in turn, forms the eastern component of the Ring of Fire, the circum-Pacific region of volcanic and seismic activity. They all share a common tectonic genesis, having formed subsequent to the break-up of the supercontinent of Pangaea. 

Concomitant with the opening of the Atlantic Ocean, the Pacific initiated consumption along the Ring, as one global ocean widened at the expense of the other. As the North American and Eurasian and South American and African plates diverged, North and South American were driven westward, all as the Farallon plate initiated subduction beneath the North America plate. In South America, the Nazca plate - a Farallon fragment - descended beneath the South American plate, uplifting the Andes including the Paine Cordillera. Further south, the Antarctic plate was eventually involved in the process. 

Cordillera del Paine's mafic and felsic magmas formed a laccolith at a shallow depth of 2-3 km some 12 million years ago in the Miocene. During ~90,000 years of emplacement, the mushroom-shaped molten mass intruded basin deposits of Cretaceous-age continental and shallow marine sandstones and mudstones, elevating the overburden into a dome. The Torres de Paine Intrusive Complex was exhumed in the Neogene and glacially carved in the Quaternary into the spectacular "Towers of Blue" we see today.

Rounding the Horn at the End of the World
Cape Horn and the Drake Passage
Southernmost Headland of the South America
Go there: 55°59′04.93" S, 67°16′22.87" W 

"I am the albatross that waits for you at the end of the world.
I am the forgotten souls of dead mariners who passed Cape Horn."
English translation of first stanza of poem by Chilean Sara Vial on a marble plaque at Cape Horn

With the ship's bell ringing loudly to celebrate our rounding of the Horn, this was our view from the Chilean Stella Australis.

Contrary to what we learned in school, Cape Horn is not the southernmost point of South America. That geographical distinction belongs to Chile's Diego Ramirez islands. The Cape IS the southernmost headland of the continent. It's also the northern boundary of the Drake Passage between the South Shetland Islands of Antarctica some 500 miles south. The notoriously rough waters and westerly gales are where the Atlantic and Pacific meet. The "sailors' graveyard" has claimed more than 1,000 ships and over 10,000 lives. In March, we "rounded the Horn" on the Stella Australis on the roughest seas the captain had seen in 35 years of service. It was a quite memorable experience.

The Drake Passage was closed some 41 million years ago when South America and Antarctica were unified and much warmer without an ice cap. That changed when the continents rifted apart and with the introduction of the Scotia tectonic plate that formed a collage of deep basins and a volcanic island arc. Once open, the Antarctic Circumpolar Current formed, the largest ocean current in the world with 600 times the flow of the Amazon. The result was Antarctic glaciation, triggering of Oligocene global cooling and the confirmation that ocean currents play a part in climate change. As for the Cape, it's part of the South Patagonian Batholith that formed from the amalgamation of subduction-related plutons during the Andean orogeny from the Late Jurassic to Neogene.

By the way, there are two famous interoceanic passages within the archipelago of Tierra del Fuego at the tip of South America. But, unlike the Drake that is "open water", the Beagle Channel and the Strait of Magellan are fjords that are too narrow for large ships and frequently icebound with headwinds often too great for sailing vessels. In common with the Cape, they were created during Andean orogenics and carved by repeated Pleistocene glaciations.

A "Warm" Remnant of the Southern Patagonian Ice Sheet
Glaciar Águila
Agostini Fjord, Tierra del Fuego, Chile
Go there: 54°28′42.06" S, 70°26′38.88" W

Glaciar Águila 
With bedrock exposed at the glacier's terminus, the ice front is stranded above a tranquil rock flour-infused proglacial lake that formed during Aquila's retreat. Bound by a system of moraines (accumulations of till), a lazy meltwater stream drains across a broad outwash fan that leads to the fjord that contains occasional chunks of rafted ice. Aquila's intense blue is due to dense ice absorbing long wavelengths such as red and transmitting scattered blue short ones. The fluted appearance is due to melting, and the corrugated furrows are aligned parallel to the direction of ice movement.

Similar to ice sheets of the Northern Hemisphere, South America's high latitudes were covered by the thick Patagonian Ice Field during the Pleistocene and began to melt after the Last Glacial Maximum some 21,000 years ago. Changing climatic conditions during the ensuing interglacial period, accelerated by anthropogenic changes to greenhouse gases, resulted in ablation (sublimation and melting of snow and ice) that exceeded accumulation. Fluctuations in the Patagonian Ice Field in the southern part of South America have contributed to the debate on climate change.

The end result has been the retreat of the once expansive ice field. Vestiges remain in the form of the Northern and Southern Patagonian Ice Sheets and a large number of alpine (mountain) and outlet glaciers. The latter, in Chile, either reach the Pacific via glacially carved fjords or the Atlantic via Patagonian lakes and rivers that empty eastward. Águila is an example of the former that empties into the Agostini Fjord located within the maze of Tierra del Fuego islands in the Magallanes region of southern Chile.

Panoramic Photo of Aquila and the Smithsonian Travelers Entourage

Currently extensive in Greenland and Antarctica, continental ice sheets have cold bottoms and remain frozen to the bedrock, whereas, alpine and outlet glaciers are on the move. Spawned by ice sheets such as the Patagonian, they are the predominant sculptors of the landscape. Although the Águila is retreating, it's fed above the equilibrium line where precipitation contributes to its growth. It's nestled on and confined by scoured late Paleozoic to early Mesozoic metamorphosed rocks overlain by Jurassic volcanic and Cretaceous clastic sedimentary rocks of the Cordillera Darwin range of the Austral Andes. 

A Sure Sign of New England Spring
Boston, Massachusetts

The male robin takes his turn at tending to the brood of hatchlings.

What could be more convenient for nature photography than a robin's nest right outside my window? In May, two red-breasted American Robins sat on four eggs for 12 days. Rather than a bed for sleep, the nest is a well-insulated incubator, which is why neither rarely left it for more than 10 minutes at a time and repeatedly turned the eggs for even heating. Their sleep, which is actually torpor (reduced metabolism and consciousness), was accomplished by roosting on a nearby branch with "one eye open" for predators.

The female's plumage is subdued compared to the male's rich coloration. Darwin recognized that sexual dichromatism facilitates mate selection (and natural selection, if it includes something like horns) but also protects camouflaged females during incubation from predatory cats, squirrels, chipmunks, hawks, jays and owls. Why are the eggs blue? The pigment may confer a biophysical advantage to the eggs by striking a balance between harmful UV radiation and beneficial IR warmth during brooding. 

Evolution has also adjusted the timing of the breeding season to maximize the number of young produced. In New England, robins mate and hatchlings emerge when food is abundant in May. Their appearance coincides perfectly with that of tasty lime green, foliage-eating Geometrid caterpillars that appear by the gazillions and dangle from the canopies of oaks and maples on long silken threads. Robins are an altricial species with unfeathered hatchlings that require intense care and feeding versus precocial chickens and ducks that are born feathered and soon on their own. Unfortunately, I missed the magical moment when the fledglings left the nest, which will likely remain empty since the weather-beaten, parasite-ridden, feces-laden nest won't be used again. 

Iceland's "Golden" Waterfall
Gulfoss National Park
South-central Iceland
Go there: 64°19′34″ N, 20°07′16″ W

"Kemst þó hægt fari."
You will reach your destination even though you travel slowly.
Icelandic proverb

The Second Tier of Gulfoss and Gorge on Strike Downfalls with Paleo-terrace Above
Facing downstream, the fall's second aligns with the basalt-walled gorge below the falls. On the far side there appears to be a paleo-river terrace that may have formed in the early stages of downcutting before the formation of the falls and gorge.

The "Golden Falls" name is derived from the color of its glacial sediment in sunlight, something we didn't see in Iceland. It's the most famous of Iceland's countless waterfalls and part of every Icelandic Golden Circle tour along with Þingvellir, a region of seafloor-spreading on land, and Geysir, that lends its name to all spouting hot springs. River Hvítá's waters flow over the falls and originate from Langjökull, Iceland's second largest glacier to the north. The genesis of the falls may have been triggered by a jökulhlaupI, which is an outburst flood produced by subglacial volcanism, or formed from meltwater generated during the glacier's retreat at the end of the Younger Dryas period of cooling that occurred during Holocene warming ~11,700 to 10,500 years ago. 

The "White River" plummets 32 meters over Gulfoss's two main steps of erosion-resistant lava and continues within a 70 meter-high, columnar basalt-walled gorge called Hvítárgljúfur. The upper step is a thin flow that overlies a thick, conglomeratic sedimentary sequence, and the lower step is a thicker flow. They differ in orientation to each other and the trend of the gorge, related to two strike-slip (horizontal) faults typical of the South Iceland Seismic Zone. The gorge's linearity is on strike with the second step and then parallels the normal fault zone (vertical movement) of Þingvellir. The faults offer a path of weakness that Hvítá has exploited and account for the zig-zag geometry of the falls and gorge. 

In the early 20th century, foreign investors wanted to harness Gulfoss within a dam and hydroelectric plant. Construction was prevented in part by Sigríður Tómasdóttir, the daughter of the land owner. To protest, she walked barefoot 75 miles to Reykjavik on unpaved roads and threatened to throw herself over the brink if the project was initiated. Icelanders are a hardy breed, intensely dedicated to resource preservation. Fortunately, the falls is in safe possession of Iceland as Gulfoss National Park.

Hrafnabjörg - A Classic Table Mountain
Central-Southwest Iceland 
Go there: 64°16′ 21″ N, 20°55′ 18″ W

Hrafnabjörg as seen from the Southeast
Lava flows cover the top of the mountain and rest on thick layers of hyaloclastites. About 11% of Iceland's total area - some 100,000 sq km - is currently covered by glaciers and ice caps, but during the Last Glacial Maximum at the end of the Ice Age, the entire country was covered as was northern Europe and northern North America. It explains why table mountains are so prevalent across the rift region of central Iceland around the Neovolcanic zone during the Pleistocene and why they are less eroded, being younger.

Only some 25 km from Reykjavík, Þingvellir is of great geological, historical and cultural significance. I've been there twice - once in a blinding snow squall and the other in fog and rain - which is a brief commentary on the country's weather. The geology is of seafloor spreading on land, where the North American and Eurasian tectonic plates are drifting apart at about 0.6 cm/yr. The event has been going on for ~200 million years in the North Atlantic, although Iceland formed only 16 to 18 million years ago.

Rupturing of the crust produced a number of NNE-SSW parallel grabens (German for "grave") on strike with the plate boundary. Extension has permitted the emanation of successive flows of lava from fissures that blanketed the graben and built volcanoes. Þingvellir is circumscribed by volcanoes that belong to four active volcanic systems:  Prestahnúkur and Hrafnabjörg on the north and Hengill and Hrómundartindur to the south.

Hrafnabjörg is a classic table mountain or tuya that formed subglacially during the last glacial period some 20,000 years ago when basaltic lava emanated from an elongate fissure. The flat-topped, steep-sided, ridge-like volcano is the most common of a number of volcaniforms in Iceland. Subglacial and submarine (water confines and cools molten magma as does ice) volcanoes may initially form a móberg ridge over a fissure or cone over a vent. A table forms, if the edifice remains confined as the lava quickly cools. 

Submarine eruptions may form islands, and subglacial ones may trigger catastrophic outburst floods and release great volumes of steam and water. Rocks are typically breccias (fragmented rock), hyaloclastites (water-induced aggregate of glassy fragments), pillow lavas (submarine basaltic eruptions in rounded heaps) at the base and layered lava flows across the summit. Hrafnabjörg's source fissure is related to the developing Þingvellir graben and is on strike with it. Use the above coordinates to locate it, and you will see this.

Hutton's Section and "Deep Time"
Arthur's Seat
Edinburgh, Scotland
Go there: 55°56′34.36″N, 3°09′59.77″ W

"The result, therefore, of our present inquiry is, 
that we find no vestige of a beginning, no prospect of an end.“
James Hutton, 1788

Yours Truly at the Basalmost Exposed Portion of Hutton's Section
At the base of Hutton's Section at Salisbury Crag on Arthur's Seat, a small block of cementstone has prised upward by the intrusion of a wedge of teschenite. The interface of the two rocks types exhibits contact metamorphism forming trachyte, a basaltic glass. At several locations within the sill, xenoliths (displaced) of cementstone are to be found.

Of all places, about a kilometer from the center of Edinburgh lies one of Scotland's most famous geological localities that has been instrumental in helping form the ideas of modern geology. It's within 640-acre Holyrood Park near the 251 meter-high summit of Arthur's Seat, which was formerly a royal hunting park and now open to the public. The Seat is the largest of three volcanic plugs (solidified vent magma) within the city that date back to the Carboniferous Period ~340 million years ago. The second is Castle Rock that leads to Edinburgh Castle, and the third is small, monument-topped Calton Hill nearby. Ice Age glacial sculpting has created their distinctive geomorphology of crag (rocky hill) and tail (glaciated ramp). 

In the Devonian, the Old Red Sandstone continent (British term for sedimentary rocks of North America's northeastern seaboard, Great Britain and Scandinavia) was built primarily via a collision of Laurentia (North America's Precambrian cratonic core) and the continent of Baltica (Europe, Scandinavia and Siberia). The event uplifted the Caledonides Mountains of northern Britain and Scandinavia. In the Permo-Carboniferous, the South Hemispheric megacontinent of Gondwana collided with Laurentia at the expense of the intervening Rheic Ocean. Oceanic plate subduction triggered back-arc extension within the overriding plate. The extensional, fault-controlled basin that formed had the structure of an ancient rift valley with sedimentation, basaltic magmatism and sill intrusion of sediments in the Midland Valley region of central Scotland. Arthur's Seat represents the time-eroded and Ice Age glaciated remnants of a volcanic plug where magma lithified within its vent. 

My Friend Tony and I Atop Arthur's Seat
Magnificent views of the Midland Valley, farther Highland Mountains, the Firth of Forth and Edinburgh are to be had from the greatly eroded and glaciated summit of Arthur's Seat on Lion's Head. 

Our sought after locality is Hutton's Section at the cliffbase of Salisbury Crags. The Section is composed of a granular limestone called cementstone (the granular carbonate equivalent of mudstone) that deposited within the aforementioned basin, while the Crag is a columnar-jointed, transgressive sill of teschenite (a type of igneous intrusive rock). In the mid-1700's, geologist James Hutton observed that the deposition of sedimentary rocks and the subsequent emplacement of the sill's molten igneous rocks must have occurred at different times and even in a different manner than geological wisdom at the time dictated. It was a ground breaking concept (pun intended) for a planet thought to be 7,000 years old - biblically speaking. 
Eroded "Bridal Path" Dike of the Franconia Range
The Franconia Subbrange
White Mountains of New Hampshire
Go there: 44°09'23.31"N, 71°39'49.15"W

Looking Almost Straight Up at the Eroded "Bridal Path" Dike
The extremely steep and slippery, eroded dike on the Bridle Path Trail leads to the summit of Mount Lafayette. The host rock is Conway granite, a medium- to coarse-grained, pink, biotite-bearing two-feldspar granite. Lafayette lies on the Franconia Ridge trail, which is a small leg of the Appalachian Trail that runs some 2,200 miles from Georgia to Maine. It's also part of the Franconia Loop, which is one of the east coast's most popular and scenic hikes.

The White Mountains reside in northern New Hampshire and a tad of western Maine. It's part of the Northern Appalachians and in turn, that of the 2,400 km-long Appalachian Mountain chain from the Canadian Maritimes to Alabama with disconnected portions in Arkansas and Oklahoma. The "Whites" includes the Presidential Range and five subranges such as Franconia, the second highest range in the system. 

The Appalachians began to form 480 million years ago but continued to grow incrementally as fragments of disassembled Rodinia and Gondwana successively reassembled during the Paleozoic (which includes the accretion of intervening oceanic lithosphere). Five phases (up from the traditional three) are assigned to the protracted event: Ordovician Taconic; Silurian Salinic; Devonican Acadian; Devonian to Carboniferous Neoacadian; and Permian Alleghenian. Each added crust to and uplifted mountains along the eastern continental margin of North America. It culminated in the formation of Pangaea and the Appalachians on strike with the zone of tectonic convergence. 

In New Hampshire's White Mountains, sedimentary and igneous rocks of the Taconic phase are found along the Vermont border to the east and are the oldest rocks. Progressing east across the orogen, Silurian marine and finally sedimentary Devonian rocks of the successive phases are encountered. The latter represents the Acadian orogenic collision of the Avalonian terrane that uplifted metamorphosed sands and muds of the Presidentials. So, how did the Franconia Range come to form later in the Mesozoic with different rocks types and to the west of the older Presidentials? What's this got to do with the "Bridal Path" dike?

My son Will and I at Galehead Hut on the Appalachian Trail of the White Mountains

Beginning in the early Mesozoic, Pangaea began to break apart. The event formed the Atlantic Ocean and present-day continents of the Atlantic realm. When Pangaea's lithosphere began to rift apart, rift-parallel grabens developed up and down the newly-active continental margin and the White Mountain Plutonic-Volcanic Suite emplaced in New England and Quebec fed by an extensive, subterranean plumbing system of dikes. As a result, most of the White Mountain's rocks are Early Jurassic granite plutons, the source of the Granite State's moniker. Many of the dikes fed the plutons or were part of ring-dike systems following the plutons' collapse into calderas. 

The "Bridal Path" dike eroded out from Franconia's Conway granite, the host rock in which it and others emplaced. Pleistocene glaciation has since sculpted the region and excavated and polished a number of erosion-voided dikes turning them into slick sluiceways for runoff such as the one in the photo. Just think of all the large-scale global tectonics and geological processes that resulted in the formation of this seemingly insignificant and largely unnoticed landform!

Will in the Bunkroom of the Galehead Hut
Sometimes, after a long day of climbing, you're still not finished. 

A Sentinel Butte with an Uplifting Story to Tell
Red Butte
Coconino Plateau, Northern Arizona
Go there: 35°49′14″ N, 112°05′23″ W

Red Butte seen on Approach from Arizona State Route 64
It's known to the indigenous Havasupai as Wii'i Gdwiisa, meaning "clenched fist mountain" and is regarded as a sacred site, one of many on the southern Colorado Plateau. The insulberg is held up by a cap of basaltic lava.

Begging to be climbed for the great view from its 912 foot-high summit, the sombrero-shaped sentinel lies about half-way to the Grand Canyon from the San Francisco Volcanic Field north of Flagstaff. Just off the highway, it rests on Middle Permian Kaibab limestones that extend to the canyon's South Rim. Above the base, Red Butte consists of Mesozoic-age brick-red Early Triassic Moenkopi mudstones followed by Late Triassic basal Shinarump Member sandstones of the Chinle Formation. 

The butte owes its existence to a remnant cap of dark gray, basaltic lava that protected it from erosion since its emplacement some 9 million years ago. The flow likely originated from a vent in the San Francisco Volcanic Field perhaps 50 miles or so to the south. The subtle tilt of the Colorado Plateau sent the low silica, low viscosity lava far from its source, but its trail across the landscape has long since eroded away along with undoubted other flows that never made it this far north.

San Francisco Volcanic Field Facing South from the Summit of Red Butte
During its six million year history, the field has produced more than 600 volcanoes over an area of 1,800 square miles. The volcaniforms range from cinder cones, lava domes and the majestic caldera-collapsed crater of the San Francisco stratovolcano known as the Peaks. Partially hidden below the vegetation of the Kaibab and Coconino National Forests, the field consists of many layers of lava and cinder cones. Red Butte's geological history provides clues as to the extent of erosion that has taken place across the field.

Mesozoic rocks once covered the Colorado Plateau and certainly the Grand Canyon since they were deposited at sea level. They are are found in southern Utah but are largely absent except in regions of northeast Arizona and isolated knobs like Red Butte. Under what geological circumstances caused the rocks to have been removed from the region?

The Laramide orogeny - the Late Cretaceous to mid-Paleogene tectonic mountain-building event related to progressive shallowing of subduction of the Farallon plate - is responsible for uplifting the Rocky Mountains and Colorado Plateau and creating its tilt. Uplift provided the impetus (in part) for the Colorado River system to carve the Grand Canyon and "unroof" most of the Mesozoic rocks from northern Arizona - of course with the exception of lone sentinels such as Red Butte with a great geological story to tell.

Cattle Drive on the Coconino Plateau
Northernmost San Francisco Volcanic Field
Northern Arizona
Go there: 35°35′24.27″ N, 111°34′10.66″ W

Just north of Flagstaff is the inwardly collapsed caldera and remnant peaks around the rim of the once-towering San Francisco stratovolcano. One of them is Mount Humphreys, Arizona's highest summit at 12,633 feet. The 1,800 square mile volcanic field extends many miles to the west, east and north of "The Peaks", almost half way to the South Rim of the Grand Canyon. You can't fully appreciate its size until you drive around and through it. During its six million year history, it produced over 600 volcanoes, mostly cinder cones, a few lava domes and layer after successive layer of basaltic lava flows. Its youngest volcaniform is Sunset Crater cinder cone. Its eruption date of 1066 AD tells us that the volcanic field is dormant and likely poised to awaken.

This October day some 25 miles north of Flagstaff, while exploring the field and cinder cones on the northern flank (and eventually climbing 71,000 year-old, 3,900 feet-high SP Mountain), I was four-wheeling my way across the Coconino Plateau on a dusty dirt road. The landscape gently undulates with Holocene to Middle Pliocene basaltic lava flows that variably blankets Early Triassic Moenkopi mudstones and Middle Permian Kaibab limestones. 

I was forced off-road to skirt a large herd of Herefords on the expansive Babbitt Ranch. Around this time of year, cowboys trail cattle from their summer grazing lands on the north side of the Peaks to winter refuge on the east side. Looking back at the herd, I chanced this fortuitous shot. It's time like this that have taught me to never get caught without a camera on the Colorado Plateau (or anywhere for that matter).

Trekking the Geology of the Tonto Platform
Grand Canyon National Park
Northern Arizona
Go there: 36°5'25.45"N, 112°7'53.45"W 

"Although this trail constrains itself to one geologic layer for most of its length,
it is anything but monotonous."
From "Hiking the Grand Canyon's Geology" by Lon Abbott and Terri Cook, 2004. 

The Gently Undulating Tonto Platform is full of "ups and downs" and "ins and outs."
The Tonto Platform, with the Tapeats Sandstone at its base, lies directly above the Great Unconformity, the monstrously long, global time gap when Rodinia fragmented apart and rifted continents drifted across the globe forming tranquil margins on their new shores.

The break-up of supercontinent Rodinia in the latest Proterozoic gave rise to a plethora of continents large and small and the consequent opening of intervening oceanic basins. The event left Laurentia, the ancient, stable cratonic core of North America, the largest domain, although assembling Gondwana would soon dwarf it in size. Rifting created passive continental margins along Laurentia's periphery. For hundreds of millions of years beginning in the Cambrian, its western shore (present coordinates) was characterized by the absence of tectonic and volcanic activity, while gradual subsidence provided accommodation space for massive deposition. During this tranquil time, the Grand Canyon's horizontal stack of layers were deposited at or near sea level and are revealed today in the colorful slopes and cliffs that tower to its rim.

With fits and starts, fluctuating Panthalassic (proto-Pacific) seas advanced ever-eastward, flooding Laurentia's western margin including the region of the future Grand Canyon. Reworking sediments derived from the land, the shores were blanketed with medium to coarse-grained Tapeats sands as deeper waters received Bright Angel Shale's fine-grained silts and muds, and even deeper, beyond the terrestrial sediments, Muav limestones were crystallized from the sea and built from shell fragments of newly-evolved marine organisms. These three lithologies form the Grand Canyon's Tonto Platform. It's a classic transgressive, onlapping and interfingering, sedimentary sequence that records the gradual migration of the shoreline as the sea advanced onto land.  

The establishment of the Tonto Group as an east-west, river-paralleling, broad and relatively flat geological bench occurred during the carving of the Grand Canyon. As the Colorado River system eroded into the deepening abyss, it eventually reached soft deposits of the Bright Angel Shale. The river began to meander, gradually undermining the cliffs of stronger strata and causing them to collapse and retreat. A similar canyon-widening phenomenon happened previously during the formation of the more elevated Esplanade Platform, when the river reached erodable shales of the Hermit Formation.

The Tonto Platform formed when it became stranded as the river began to chisel into the erosion-resistant deposits of the Granite Gorge, Rodinia's basement suite of metamorphic Vishnu schist and igneous Zoroaster pegmatite granites. The platform, therefore, survives as a mid-canyon paleo-terrace some 4,000 feet below the rim and some 1,000 feet above the river (both on average). 

Yours Truly on the Tonto Trail at Sunrise
The 95 mile-long Tonto Trail - one of the classics of the Southwest - follows the platform largely on the Bright Angel Shale and is anything but flat (and easy). It has seen use as a transcanyon route by everything from indigenous native Americans to miners, feral burros and adventurous, overnight backpackers where shade is rare and water is scarce (depending on the season). The trail rises and falls with the platform's dramatic undulations and swerves in and out to follow the innumerable massive drainages that repeatedly punctuate it from the rims. Photo by geologist, author and guide Wayne Ranney. Shameless plug - Wayne's and Ron Blakey's newest book is out "Ancient Landscapes of Western North America" by Springer Publishing (here). 

Well, that's it for 2017. 
Thanks for following and contributing to my blog. 
I'm humbled by your comments and most appreciative of your visits. 
Have a Happy and Healthy New Year! See you in 2018.

Will during our Ascent of Mount Lafayette in the Franconia Range of the Whites