So it is said that sedimentary rocks are stratified. Much of the North American continent is mantled with sedimentary rocks. They have been raised from the floor of the sea where they were formed on top of the older igneous rocks that mark the real crust of the Earth. Sedimentary Rocks One of the commonest sedimentary rocks is sandstone. Shale is cemented mud, silt, or clay.
Conglomerate is a coarse rock made up of pebbles and even boulders.
If the large fragments still are rough and angular, the rock is called breccia. Geodes are hollow spheres in limestone. They have an outer layer of mineral material and are usually lined with quartz crystals projecting from the inner rim toward the hollow center. They may originate as an internal cavity, such as the unfilled space within a fossil, or may expand from pressure. The third group of rocks is the metamorphic.