Shale is a Sedimentary Rock.
Sedimentary rock is one of the three main rock families. Sedimentary rocks are made from sediment. Sedimentary rocks cover 75-80% of the Earth. You will mostly find sedimentary rock where there was once water. That is because sediment carries the sediment and drops it on top of other sediment. Over time, the sediment builds up and the pressure of each layer of sediment compresses into a sedimentary rock. If you closely examine a sedimentary rock, you will see that it is built up of layers.
Sediments such as small rocks and the remains of living things are deposited in the sea by rivers and waves. These sediments build up layer by layer and after thousands or millions of years, they forming thick deposits on land or on the sea floor. The weight of the sediments on top compact those below. Over time, they form sedimentary rocks.
Cementation is the process of sediment being stuck together like cement. How this works is that water comes in and the sediments squeeze together. This will cause the water to be forced out by the pressure. The water is then replaced by other minerals to hold all the sediments together to form a sedimentary rock.
If you look inside of Sedimentary Rock, you may find the remains of a once-living organism! They are also known as fossils. The reason for that is because the skeleton of the dead animal laid there as sediment. As time passed, new layers were put on top of the bones. Those bones were compressed and made part of the sedimentary rock just like any other new layer.