Little bits of rocks are broken away by rain, rivers, or wind, these pieces may travel over land and finally settle down. When these settle down they exert pressure in the rocks beneath it. Soon more rocks and pieces (sediments) of dirt will pile on top of the first layer. Soon this dirt and rocks will form silt (also known as mud). When this silt gets thick enough it will place enough pressure onto the bottom layer of the silt to cement the rocks and dirt to others, making sedimentary rocks.