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Chapter 7: A Surface Veneer: Sediments, Soils, and Sedimentary Rocks

Feature Articles

Geology You Can See: Types of Sedimentary Rock

by Stephen Marshak

Figures 1-9 credit: Stephen Marshak

Figure 1	Outcrops of sedimentary rocks can be dramatically beautiful. What makes them this way? In some cases, as illustrated by the brilliant orange-red sandstone of Monument Valley, Arizona , it's the color. The late afternoon sun can make such rocks seem to glow with warmth against the blue sky. In other cases, the beauty lies in texture. For example, in cross-bedded sandstone, each bed contains a fabric that results from the accumulation of sand on the slip face of dunes (large and small).
Figure 2	In this example from the coast of Scotland , thin sandstone beds have been tilted during later mountain building.
Figure 3	The giant cross beds of sandstone in Zion National Park (Utah) weather out to form a mega-herringbone texture on the face of outcrops. The girl has posed just above a master bed plane, which truncates the tilted portion of the underlying cross-bedded set.
Figure 4	The size of these cross beds indicates that they formed from sand deposited in large desert dunes. Similar desert dunes make up the large ledge of sandstone that protects the ancient Native American village at Mesa Verde, Colorado.
Figure 5	Most limestone units consist of calcite that was extracted from the ocean by living organisms. For example, the coral of this coral reef in Australia represents limestone in the making. Some limestone is made up of finer-grained fragments (even as small as mud) from shells that have been broken up or from skeletal flakes of certain species of plankton.
Figure 6	Once deposited, calcite recrystallizes to form a gray mass, as illustrated by this exposure of Devonian limestone in central New York State.
Figure 7	The grain size of sedimentary rocks, and the sorting of sediment, tells us about the depositional environment in which the sediment accumulated. For example, the siltstone and shale at the base of the cliffs in Monument Valley probably accumulated in a river system. The massive cliff of sandstone is the remnant of ancient dunes. Figure 7 shows a boulder of conglomerate from a Precambrian sedimentary unit in Norway. The rounding of the clasts suggests that they tumbled around in a high-energy stream. The stream water washed away most mud and sand, leaving this rock fairly well sorted.
Figure 8	In contrast, the glacial till exposed in this cliff on the west coast of Ireland is poorly sorted - boulders, sand, and mud are all jumbled together. That's because the till was deposited by a glacier, and glaciers are made of solid ice and thus can carry sediment grains of any size.
Figure 9	Fossils can be preserved in sedimentary rocks. Figure 9 shows two examples. The fossil in the black shale, which comes from a coal-bearing sequence of strata in Pennsylvania, represents a Carboniferous-age fern. The sandstone block contains the fossil of a Cretaceous fish, which swam in a long-gone lake in central Brazil.