| SEDIMENTARY ROCKS |
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Sedimentary rocks are formed from pebbles, sand, silt, or clay. They are made from the end products of erosion, which breaks down rock into tiny pieces. Streams carry the tiny eroded fragments of stone to lakes and oceans, or spread them out over the flood plains of of rivers. Layers of these materials are built up over millions of years. More and more sediment is added. Dissolved minerals in the water are gradually precipitated, filling in the spaces between the rock fragments. The layers of sediment turn into rock strata. Sedimentary materials are are also deposited by wind or glaciers. The strata of sedimentary rocks are often colored by minerals. A reddish color, as in the picture above, is caused by iron oxide, also called rust. Notice the more distant hills: some of their strata are colored by other minerals, such as manganese.
For thousands, even millions of years, little pieces of our earth have been eroded--broken down and worn away by wind and water. These little bits of our earth are washed downstream where they settle to the bottom of the rivers, lakes, and oceans. Layer after layer of eroded earth is deposited on top of each. These layers are pressed down more and more through time, until the bottom layers slowly turn into rock.
Sedimentary strata formed
A stream is carrying silt and clay into a large lake or sea.
Millions of years have gone by and there are many layers of deposited sediment now. This part of the earth has been sinking, and the sea is much larger now.
More sediment has been deposited. The layers on the bottom have a lot of weight on them. They will heat up from the pressure.
Pressure from underneath the sediments is causing them to bend. The rock is so hot that it is somewhat plastic, and so will bend slowly if enough force is acting on it.
A big crack has occurred in this huge block of sediment. One block slid downwards past the other. This crack is called a fault. The sliding produced an earthquake. You can see the layers are no longer lined up.
Additional earth movements have stretched and bent the rock layers even more. One end of the layers is above ground, and it shows a very tilted layer stripes. One can see examples of bent and tilted layers in many places in the world. Sometimes the strata run straight up and down. Scientists learn about the age of the earth from studying these strata.
Sedimentary rocks & fossil
Fossils are generally found in sedimentary rocks formed by soft silts and muds. The soft sediment preserves the fine details in the bones, teeth, and leaves of plants. Sometimes sediments fill an opening in a bone or shell and leave behind a cast of the inside of the living thing. Plants are often fossilized in soft sediments which preserve the structure of the veins in the leaves.  |
