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of feet of sediment were removed from the landscape, including most of the Grand Canyon Supergroup strata and some depth of the basement rocks. These changes created the Great Unconformity, the boundary between the Grand Canyon Supergroup and the overlying Paleozoic sedimentary rocks.

      Today the only Grand Canyon Supergroup strata that are preserved in the vicinity of the Bright Angel and South Kaibab trails are those on a down-dropped block below the Tipoff, termed Cremation Graben (German for “grave”). And even here, only the three lowermost strata, the Bass Limestone, Hakatai Shale, and Shinumo Quartzite are preserved.

       GEOLOGY DETERMINES WHERE THE TRAILS ARE LOCATED

      If you visited vista points on the South Rim before embarking on your hike, you likely stared at the steep Kaibab Formation that forms a 350-foot-high cliff just about everywhere, providing few locations to descend below the rim. The Coconino Sandstone and Redwall Limestone form similarly impenetrable barriers. The Bright Angel Trail follows the Bright Angel Fault: Movement along the fault broke the solid rock, allowing erosion to proceed more rapidly. Eventually steep talus piles formed along the fault zone, allowing passage through otherwise vertical cliffs. In addition, the faulting has caused the rock on the southeast side of the fault to be about 200 feet lower than that on the northwest side, and the trail can snake back and forth across the fault depending on which side provides easier passage. The benefit of the fault scarp is especially visible where the Bright Angel Trail cuts through the Coconino Sandstone above the 1.5-Mile Resthouse and along Jacobs Ladder below the 3-Mile Resthouse. This fault first formed during the breakup of Rodinia 750 million years ago.

      In contrast, the South Kaibab Trail is predominantly a ridge route that exploits locations where the normally steep rock layers have begun to erode, because they are outcropping along a narrow ridge. And here too, the passage through the Redwall Limestone follows a small fault.

      Passive margin, 525 million to 320 million years ago: The breakup of Rodinia created a “passive margin,” or tectonically quiet region, along the then western edge of North America, the Colorado Plateau region. Along such margins the seafloor often sinks rapidly continually creating more space for sediment to accumulate and allowing thick rock strata to form. The prominent striped layers in the Grand Canyon, a series of strata 4000 thousand feet thick, were deposited during this tectonic regime with the type of sediment changing with shoreline position and water depth. (Up to 18,000 feet of sediment was deposited on the Colorado Plateau, but only the lower strata are preserved in the Grand Canyon.)

      The oldest (and lowest) of the layers is the Tapeats Sandstone, formed from former beach sands. The Bright Angel Shale, formed in shallow offshore waters, follows; the mud-sized particles comprising the shale were carried a short distance out to sea before being deposited. The Muav Limestone, the third layer, is constituted of a combination of calcite that precipitated and calcite-bearing shells that were deposited. Together, these three layers represent a 20-million-year period of rising sea level: While sand-sized sediment was amassing at one location, silt was being deposited in shallow water to the west, and calcite was accumulating even farther west. As the sea level rose, each environment shifted eastward, such that the three sediment types overlay each other in the Grand Canyon.

      No records remain from the following 120 million years, probably because decreasing sea level at the end of this period allowed the upper sediment layers to erode. One intermediate layer, the Temple Butte Limestone, is present as a thick stratum in the western Grand Canyon, where waters were deeper, but along the Bright Angel and South Kaibab trails it exists only in eroded channels in the Muav Limestone. Note that each of the Grand Canyon’s rock layers above the Muav Limestone is separated by an unconformity; some gaps in the rock record are brief, but others correspond to the removal of considerable sediment.

      When rock strata abutting one another do not represent a continuous time sequence, the surface between the two layers is referred to as an unconformity. This gap in time indicates that sediment was eroded from atop the lower stratum before the upper stratum was deposited.

      By 340 million years ago sea level was again rising, and much of the Colorado Plateau region was submerged beneath a large, shallow sea. Rivers transported little sediment to the region, creating the clear water environment that promoted the deposition of the thick layer known as the Redwall Limestone. The sea then retreated, eroding the top of this layer.

      Passive margin, but tectonic collisions to the east as Pangaea forms, 320 million to 250 million years ago: Around 320 million years ago the supercontinent Pangaea, a landmass composed of all continents with North America along its western shore, began to form. The continental margin west of the Grand Canyon was still passive, but to the east, an ancestral mountain range, the Ancestral Rockies, rose. As this mountain range was uplifted, large quantities of sediment were eroded, first filling large basins immediately west of the mountain range, and later spilling westward to the Grand Canyon region. It was a desert environment all the way to the coast.

      The Supai Group is composed of sediments from 320 to 285 million years ago. It is primarily red desert sands from the eroding mountains to the east. This sediment was deposited in an extensive coastal plain with enormous river deltas. During this time period Pangaea was centered over the South Pole and recurring glaciations (due to changes in sun strength) tied up vast quantities of water. As a result, sea level fluctuated more than 400 feet every 100,000 years, leading to repeated incursions of the sea, creating thin deposits of shale and limestone between the layers of desert sand.

      The environment was similar when the next stratum, the Hermit Formation, was deposited, except that sea levels were lower and the formation is exclusively terrestrial. Large rivers continued to carry red mud and fine sand from the deserts to the east. As the continent became ever drier and the shoreline retreated farther west, large dunes spread across the Grand Canyon area. These giant sand dunes are preserved as the Coconino Sandstone (275 million years ago).

      While the inland drought was continuing, the shoreline crept east again: By 273 million years ago, a very shallow sea again covered the area, leading to the deposition of intertidal deposits (the Toroweap Formation) and then deeper water where calcite accumulated (the Kaibab Formation; 265 million years ago). The Toroweap Formation contains gypsum and salt crystals, minerals formed by the evaporation of water.

      This marks the end of the sequence of sedimentation that is preserved today—the 4000-foot-thick sequence of multicolored strata that makes the Grand Canyon so spectacular. However, the subsequent 250 million years are important as well: Those rocks needed to be lifted far above sea level and carved by flowing water. The remainder of the geologic description is therefore focused on the sequence of events that led to the creation of the Grand Canyon.

      Initial breakup of Pangaea, 250 million to 145 million years ago: The Grand Canyon region was an arid terrestrial environment throughout this time. The 4000 feet of sediment, mostly desert sand, deposited atop the Paleozoic strata have since eroded. No sediments from this time period, the Mesozoic, survive in the Grand Canyon, but they are visible farther north on the Colorado Plateau.

      Subduction of the Pacific Plate begins, 145 million to 70 million years ago: During this period, Pangaea continued to breakup, causing North America to be pushed westward, and the Pacific Plate to begin subducting beneath it. This new subduction zone created the beginnings of California’s Sierra Nevada and thickened the continental crust west of the Grand Canyon, causing the Colorado Plateau to be depressed. An internal sea, the Mancos Sea, flooded the central parts of North America, including much of the Colorado Plateau. The water retreated toward the end of this period. Around 70 million years ago, the strata that are now on the rim of the Grand Canyon were still at sea level.

      Subduction expands eastward, 70 million to 18 million years ago: Around 70 million years ago the effect of the subduction zone at the western edge of North America began to be felt much farther east, leading to rapid uplift inland. The uplift includes the Laramide Orogeny (the uplift of the Rocky Mountains), the formation of the Mogollon Highlands in southeastern Arizona, and the uplift of the Colorado Plateau. In the Grand Canyon, the outside pressures caused the buried rock strata to be uplifted, raising the rocks we see today far above sea level. In order for the Colorado River to later carve such a deep canyon, the strata needed to be elevated; an ocean-bound stream can,

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