![]() Certain fossils, called index fossils, are particularly useful because they are abundant in a relatively narrow time range. Sedimentary rocks often contain fossils that can be used to help identify the age of the rock. Examples include: sandstone, mudstone ,shale, siltsone, limestone, and more. Sedimentary rocks are formed by smaller pieces of sand and mud stick together in layers. Granite (cooled from magma, known as an intrusive igneous rock) and basalt (cooled from lava, known as an extrusive igneous rock) are two types of igneous rocks. Igneous rocks are cooled magma (melted rock found underground) or lava (molten rock found above ground). The three main types of rock are igneous, sedimentary and metamorphic. In the Grand Canyon, unconformities are common in the Grand Canyon Supergroup and the Paleozoic Strata. Missing layers may seem like a problem, but the very fact that there is this gap in the record provides information to geologists, indicating changing ocean levels or changes in the Earth’s crust. You can think of unconformities as missing “pages” in the book of the geologic record. New sediment eventually forms new rock layers on top of the eroded surface, but there is a period of geologic time that is not represented. Unconformities are gaps in the geologic record that occur when rocks or sediments are eroded away and time elapses before new deposition occurs. The image on the left is a recent photograph of Grand Canyon from Walhalla Plateau, with the red line showing the Great Unconformity that was first noted by Powell (credit: Annie Scott/USGS). “A” is the metamorphic basement complex (Early Proterozoic Vishnu Group), with igneous intrusives labeled “a” “B” is the Grand Canyon Supergroup (Middle and Late Proterozoic) “C” indicates the Paleozoic strata “x” and “y” delineate the major unconformable contacts. The image on the right is a stratigraphic section of Grand Canyon by John Wesley Powell (1875). These three main sets of rocks were first described by the explorer and scientist John Wesley Powell during his expeditions of the Grand Canyon in the late 1860s and early 1870s. The three main rock layer sets in the Grand Canyon are grouped based on position and common composition and 1) Metamorphic basement rocks, 2) The Precambrian Grand Canyon Supergroup, and 3) Paleozoic strata. This principle is a key part of determining the relative age of a rock layer. The Law of Superposition states that sediment is deposited in layers in a sequence, the oldest rocks are on the bottom and the youngest rocks are on the top, similar to the way that sand piles up in an hour glass. In the Grand Canyon, there are clear horizontal layers of different rocks that provide information about where, when, and how they were deposited, long before the canyon was even carved. Stratigraphy is the study of the rock layering, and reveals a wealth of information about what Earth was like when each layer formed. Introduction to Grand Canyon Geologic Principles Photo taken during a ceremony commemorating the 150th anniversary of the John Wesley Powell expedition. Grand Canyon National Park from Powell Point on the South Rim.
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