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.
Last modified January 6, 2004 by Lisa Gardiner.

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