et's try applying some of the basic concepts you've learned. To the right is a cross section of some of the rock layers in the Grand Canyon. Pay no attention to the terms CAMBRIAN, DEVONIAN, etc. They are names for time units we will discuss later. Instead, focus on the colored sedimentary rock units.


For the horizontally layered units the oldest layers are on the bottom and get progressively younger as we go up. Look at the section which includes the Tapeats Ss., Bright Angel Sh., Mauv Ls., Temple Butte Ls., and Redwall Ls. "Ss" = sandstone, "Sh" = shale (claystone or mudstone), and "Ls." = limestone. I know, from having been there, that the base of the Tapeats sandstone contains boulders and pebbles weathered out of the units underlying it. The fossils present all indicate marine water.

Based on what you have just learned about sediments and sedimentary rocks, what can you summarize about the environments of deposition in this area when these units were being deposited?


These four units offer a classic evidence of a transgression - a rise in sealevel that drowns the surface and then pushes the shoreline farther and farther inland, so that a given depositional site becomes father from land and the source of sediments. Now look at the overlying Supai Group, Hermit Sh. and Cocconino Ss. The fossils here indicate terrestrial or fresh water deposition. What does this represent in terms of depositional environment?


Where did the sediment come from (in terms of environment, not geography)?


All of the sandstones are composed of clean quartz sand. What does this tell you about the source?


To switch locations, let's consider the Hawaiian Islands. Recall how they were formed and what kind of rock they were formed from. Would it surprise you that some hawaiian beaches are black? What would you expect the beach sands in Hawaii to be composed of ?

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