A (Geo)History of the Grand Canyon

One of the most iconic features of the United States is the geologic glimpse into the past known as the Grand Canyon. What is essentially a big, gaping hole in the ground has become a perennial tourist favorite and a geological gold mine. The Grand Canyon is located in Grand Canyon National Park, one of the first national parks in the United States, and was a favorite hunting destination of President Theodore Roosevelt, the biggest proponent of the national park system ("Grand Canyon"). It has contributed valuable information to geologists, historians, and the general public, and within its walls are keys to the past stretching from the Precambrian to the Ancestral Puebloans.

Over a mile deep in some locations, the Grand Canyon stretches over 277 miles, literally cut through by the Colorado river, from Lee's Ferry to Grand Wash Fault ("Grand Canyon"). Although it is not the deepest canyon even in North America, its attraction lies in breathtaking landscapes and intricate geologic history ("Grand Canyon National Park, A Quick Look"). Originally garnering interest from miners in the 1800s, settlers quickly realized that tourism would be much more profitable. Eventually a railroad was built to the South Rim in 1901, a lavish hotel erected, and finally the Grand Canyon became a national park in 1919, only three years after the National Park Service came into existence.

The first European to record seeing the Grand Canyon was García López de Cárdenas accompanied by Hopi guides in 1540 (Ribokas, Human History), although the Canyon remained in obscurity until 1869, when retired Civil War Major John Wesley Powell led a scientific expedition down the Colorado river in wooden boats ("Grand Canyon," "GC, A Quick Look"). The trip took three months, and was the first known expedition through the Canyon. Powell called the sedimentary layers "leaves in a story book ("Grand Canyon")." But even before the scientific expedition of Powell and the explorations of Cárdenas, humans have been attracted to this unique geologic feature, beginning with Paleoindian visitors as far back as 10,000 years ago, with habitation definitely occurring at intervals from approximately 4,000 years ago up to the present day (Ribokas, Human History; "GC, A Quick Look").

Split twig figurines found in caves are the oldest evidence of human habitation in the Grand Canyon ("GC, A Quick Look"). These Desert Culture peoples were followed by the Ancestral Puebloans, as they prefer to be called, or Anasazi, by which name they are much more well known, who settled throughout the Four Corners region, and built their homes within the canyon walls ("Grand Canyon"). Most historians agree that the modern Hopi are descended from the Ancestral Puebloans, and now live to the east of the canyon; the Hopi belief is that their ancestors emerged from the canyon, and that the old spirits reside there ("GC, A Quick Look"). The Havasupai have also inhabited a section of the inner canyon for several hundred years. The oral histories, as well as archaeological evidence, confirm extended habitation in and around the rim and caves of the Redwall Limestone (Ribokas, Human History).

In 1869, Major John Wesley Powell led an expedition through the Grand Canyon in four small wooden boats ("GC, A Quick Look"). This trip, and a subsequent partial trip produced a detailed map and numerous papers concerning the geology of the region (Ribokas, Human History, "Grand Canyon"). The sedimentary layers he so poetically described detail a complex geologic history, from the Vishnu Schist group and Zoroaster Granite intrusions of the Precambrian era to modern day (Prothero et al., 314; "GC, A Quick Look"). The Vishnu group can be found at the base of the Inner Gorge; however, lying nonconformably on these granites and schists are upper Proterozoic and Paleozoic sedimentary units (Prothero et al., 313). A nonconformity occurs when stratified sedimentary rocks above overlay igneous or metamorphic rocks below; the surface of the igneous or metamorphic units has been eroded, and overlaid by sedimentary deposits, with missing geologic time in between (Bates et al., 348). This geologic feature resulted from structural deformation which accompanied the metamorphism of the Vishnu group (Press et al., 198). The lower Proterozoic is not represented; geologists cannot tell whether environmental forces prevented or hindered continuous deposition, or whether lower Proterozoic deposits did occur, but were subsequently eroded away. The Vishnu group is far older than the canyon itself, dating to approximately 2000 million years ago ("GC, A Quick Look").

Above the Vishnu groups lay the more well-known sedimentary units, many of which were deposited in the warm, shallow seas or littoral environments which periodically covered this region of North America ("Grand Canyon"). In some sections the Unkar and Chuar metasedimentary groups of the upper Proterozoic are tilted from their original horizontal deposition and overlaid by the Cambrian Tapeats Sandstone in an angular unconformity (Prothero et al., 313). These were once sedimentary groups; over time, pressure and/or heat have worked to partially metamorphose the rock at an extremely low grade (Bates et al., 323, 322). In other sections, the Tapeats group lies directly on top of the Precambrian Wapatai shales (Press et al., 193). Geologists call the whole angular unconformity - termed such because of the steep tilting of the bedding - between the Precambrian schists and granites and the sediments overlying it the Great Unconformity.

The sediments dating to the Cambrian include the Tapeats Sandstone, the Bright Angel Shale, and the Muav Limestone ("GC, A Quick Look"). Overlaying these is a significant disconformity (Prothero et al., 313). A disconformity occurs when there is a sequential break between parallel beds of sedimentary units, usually caused by erosion, and indicated by the existence of the missing beds in other locations (Bates et al., 142-3). For instance, key sandstone beds in the Grand Canyon can be directly stratigraphically correlated with bedding occurring in Frenchman Mountain, hundreds of miles away near Las Vegas, Nevada (Prothero et al., 318). Changes in thickness, name, or even composition of the rock according to local climate can change over distance, but geologists infer their ages from the formations, such as the Hermit Shale and the Mississippian limestones, that help to establish a temporal correlation.

There are certainly many other disconformities located in the geology of the Grand Canyon; another example is the newly named Pennsylvanian Surprise Canyon Formation (Prothero et al., 313). The Redwall Limestone eroded at some point, and combined with depositional issues, a disconformity has resulted, wherein the Pennsylvanian Surprise Canyon Formation now overlies the Redwall Limestone, thickest in the pockets of the Redwall which saw greater rates of erosion, possibly due to streams or slides.

There are numerous gaps in sequences dating to the Pennsylvanian and Permian; many geologists consider many of these gaps to be paraconformities (Prothero et al., 312-313). Not all geologists agree that paraconformities can truly be differentiated from disconformities (Prothero et al., 313); in paraconformities, the erosional surface cannot be discerned (Bates et al., 368). These breaks in sequence may be defined by other indicators which differ significantly in age, such as fossils. As a matter of fact, less than 40 percent of the Paleozoic era is represented in the strata; the rest was either eroded or was never deposited (Press et al., 199). Paraconformities are extremely common (Prothero et al., 313).

Only some of the sedimentary beds contain fossils, so that many beds can only be dated by reference to the overlying and underlying beds (Prothero et al., 315). One case where this occurs is with the Bright Angel Shale, which contains type fossils of trilobites; it overlies the Tapeats, which contains no fossils at all (Press et al., 199). Overlying the Bright Angel Shale are the Muav Limestone, Temple Butte Limestone, and Redwall Limestone, altogether representing a span of nearly 200 million years, but with major sequential gaps, recognizable by significant breaks in the faunal succession (Press et al., 198-99). The duration of just the gap of the Ordovician-Silurian-Devonian unconformity is actually longer than the combined time period represented by the rocks actually present in the Grand Canyon (Prothero et al., 313). Such is the case with all sedimentary records; even a geologically "good" example is more about what is missing than what is present (Prothero et al., 314-315).
Following these limestones is the geologically younger Supai group, composed of the Watahomigi, Manakacha, Wescogame Formations, and the Esplanade Sandstone ("GC, A Quick Look"). These date to the Pennsylvanian and Permian, and contain fossils of plants similar to those often found in North American coal beds, as well as footprints of primitive land reptiles (Press et al., 199). This indicates a depositional transition away from the shallow sea deposits and littoral conditions which occurred from the Cambrian to the Mississippian, to a swampier climate. Above the Supai group lies the sandy red Hermit Shale (Press et al., 199; "GC, A Quick Look").

One of the major depositional exceptions occurs next in the sequence. The Coconino Sandstone, located on the eastern edge of the Grand Canyon actually formed from dunes in a desert climate ("Grand Canyon"; Prothero et al., 321). Geologists infer this from the animal tracks and the cross-bedding specific to the formation of eolian, or wind-blown dunes (Press et al., 199; Bates et al., 154). Eolian dunes will yield a difference appearance in the rocks than would dunes formed by the action of water currents or waves (Press et al., 306). The formations tell more than a history of deposition or a sequence of time; they provide snapshots of paleoclimates as well. The area where the Colorado River would carve out the Grand Canyon underwent many changes in climate, although only the changes which allowed for deposition may be accounted for.

Topping the unique Coconino sandstone are the Toroweap Formation and the Kaibab Limestone. Both formations are limestones; the Kaibab is a massive cherty, sandy rock which indicates another climate change from desert to shallow sea (Press et al., 199). Cherts are microcrystalline or cryptocrystalline sediments (Bates et al., 85). Bedded chert deposits would occur in a deeper marine environment, so the Kaibab deposits indicate an environment transitioning from a coastal sandy area to a deeper area, indicated by the fining of the quartz grains (Prothero et al., 263-264). Nearest the Grand Canyon in present day the Kaibab is the uppermost rock layer present. Although the youngest, it dates to at least 250 million years old; the rest of the geologic time scale is not represented except in isolated areas, and some distance away from the inner canyon (Press et al., 199).

The other half of the Grand Canyon story is how the river itself cut through the rock layers; the river basin itself is less than six million years old ("Grand Canyon"). The Laramide Orogeny around 65 million years ago caused the Colorado Plateau to uplift, although unevenly, causing the stream gradient to change. The river changed course numerous times, but eventually this continued uplift, plus the opening of the Gulf of California around 5.3 million years ago, which lowered the river's lowest point, created the conditions necessary for the Colorado River to begin gouging out sections of the sedimentary rocks ("Grand Canyon"). Additionally, as a result of the semi-arid climate the area enjoys today, extremely dry conditions are interspersed with sudden, violent downpours, which create ideal conditions for mudslides and other powerful erosional features ("GC, A Quick Look"). In fact, prior to the creation of the Glen Canyon Dam, over a one hundred year period the Colorado River carried over three cubic miles of sediment into the Pacific Ocean (Ribokas, Geology).

The beauty and attraction of the Grand Canyon lies in more than just its visual appeal, which is truly stunning; the cultural stories told by the Ancestral Puebloans, Hopi, and Havasupai are invaluable pieces of American history, and the geologic stories are equally invaluable to the scientific community, contributing to a greater understanding of Earth's history. The large, missing section of earth called the Grand Canyon is a treasure in its absence. It took only a short time, a geologic blink of the eye, and a river to carve down to rock nearly two billion years old, showing how powerful the forces of nature can be.