Sedimentary Rocks Contain Clues to Ancient Environments

As climate changes over time, so do the environments that are present in a region. For instance, at one time a region might contain a vast lake and river system, but millions of years later there may be no trace of water at all. While water may have left the region, traces of the lake and rivers might remain in the sediment that was deposited by the water and the shape of the landscape as well. These traces allow us to figure out what environments were like in the past!

Different types of environments contain different types of sediments. The type of sediment and the way that it is deposited determines the types of sedimentary rocks that will eventually be formed. Thus, sedimentary rocks formed in a lake will be different from those formed in a desert.

Geologists look at ancient sedimentary rocks to figure out the type of environment that once was in the location where the rock formed. The table below lists environments that are common on Earth and the physical features that characterize them. Click on the pictures below to take a look at environments and the rocks that are made from them.

Environment	Sediment Size	Sediment Sorting	Structures and Features	The Environment Today:	Evidence Preserved in Rock:
Lake	Mud	Fair	Thin layers called laminations Sometimes mud cracks
Swamp	Mud		Organic material makes coal
Desert Dunes	Sand	Very good	Cross-beds Rounded grains
Alluvial Fan	Sand and gravel	Poor	Angular fragments of rock
River	Silt, sand, and gravel	Poor	Rounded pebbles, channel shape Cross-beds and ripple marks
Lagoon	Mud		Mud cracks and ripple marks
Beach	Silt, sand, or gravel	Good	Mud cracks and ripple marks Laminations and other thin layers
Shallow Ocean	Silt and sand	Good	Thick or thin layers ripples and cross-beds
Deep Ocean	Mud, with thin layers of sand or silt	Fair	Layers of mud. Thin sand layers form as sediment flows downslope.
Tropical Ocean	Sediment made of Calcite (and other carbonate minerals)	Good to poor	Most sediment comes from the skeletons of marine life.

Page created January 6, 2004 by Lisa Gardiner.
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