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Earth Geology Rock Layers

Stratigraphy: Rock Layering

Highway roadcutThe rocks of the Bryant Watershed are sedimentary. "Sedimentary" means that particles, like soil or sand, or minerals, have dropped or crystallized out of the water (or in some places were blown there by the wind). These sediments then built up in layers and turned to rock over a long period of time. Here in the watershed you find them in nearly horizontal layers. You can see this layering exposed along the bluffs and natural outcrops and most clearly in highway roadcuts (Figure 1).  This layering shows that these rocks began as layers, or beds, of mud on the floor of an ancient seabed. These sedimentary beds vary from inches to several feet in thickness. The different beds, and the lines called bedding planes that separate them, record changing conditions while sand and mud were being deposited on the sea floor. By studying this layering, geologists learn volumes about the ancient environments in which the sediments were deposited. 

If you observe the layering in the mud at the bottom of a puddle or a stream, it seems obvious that the layers on the bottom of the pile were there before the ones on top and are therefore older. This simple observation has given geologists a means of telling relative time in the history of the development of the rock layers. Those layers on the bottom are older than those on top. Therefore, this pile of layered rocks becomes a record of time - a history of the events that occurred during the formation of the rocks. 

The layering you see in a roadcut is just a small part of the overall picture. Geologists have grouped these layers into major units called "formations", which can be hundreds of feet thick. These formations can be traced or correlated across wide areas over hundreds of miles. Variations in the formations across a region tell geologists even more about the ancient environment. Geologists want to know about these ancient environments to help search for petroleum and other natural resources.