Over the course of millions of years, the igneous rocks are weathered down by forces of wind and water. Fine particles of dirt begin to cover the landscape. Often, these small rock particles end up being suspended in water, and find their way to the bottom of lakes, streams, and the ocean.
Slowly, the layer of sediment on the bottom of lakes, and especially on the bottom of the ocean grows deeper and deeper, reaching depths of thousands of feet. The weight of all the sediment becomes immense, pushing down on lower layers of sediment with tremendous force. In addition, a number of minerals, which act like cement, bond the sediment together, causing it to form sedimentary rock.