All the rocks that are exposed at the bedrock surface, however, are sedimentary rocks. Most consist of sandstone, shale, siltstone, limestone, and dolostone. Other rock types are coal, conglomerate, gypsum, claystone, and chert. Deep wells and exploratory test drillings have encountered granite, gabbro, basalt, andesite, and metasedimentary rocks at depths of 3,500 feet to about 3 miles below the bedrock surface. Groupings Sedimentary rocks occur as parallel or nearly parallel layers, or beds.
Formations are a shorthand way to describe a collection of rocks that are similar.
Typically, formations are tens to hundreds of feet thick, and they can be traced for tens to hundreds of miles. Groups are often hundreds to thousands of feet thick and can be traced hundreds of miles. The map to the right shows groups in Indiana. Groups are commonly shown on statewide maps, because formations, and especially beds, would be too thin to draw without making a very large map. Like formations, there is nothing special about the group names.