Unit 8.3-Sedimentary Rocks

Unit 8.3-Sedimentary Rocks

1. Clastic

   • Clastic Sedimentary Rocks are formed when sediments are laid down by wind and water,gravity & ice, and then compacted and cemented.
   • They contain broken pieces of rocks. These pieces are known as clasts
   • They may have layers like stacks of pages in a book because the sediments build up one on top of another as they are deposited.
   • These may contain rock fossils

2. Chemical

   • When minerals dissolved in groundwater crystalline within the sediments or when the water evaporates
   • Chemical sedimentary rocks are quite soft.
   • This helps to distinguish them from much harder igneous rocks

3. Biogenic

   • Formed when dead plants or matter accumulate
   • And then cemented together
   • These sediments are mostly from organisms but they are may be small amounts of sediments from weathered rock
   • Coal is a type of biogenic rock.

4. Relative age of Rock Strata

   • Relative age is not the actual age of the rock but only indictaeds whether a rock is older or younger than another rock.
   • Be found over a widespread area
   • Have lived in a narrow period of time in the past
   • have been abundant with lots of organisms