Kansas bedrock is old in the east (blue and purple) and young in the west (green and gold), with a long gap in age between them. The eastern section is late Paleozoic, beginning with a small portion of the Ozark Plateau where rocks date from Mississippian times, about 345 million years old. Rocks of Pennsylvanian (purple) and Permian (light blue) age overlie them, reaching to about 260 million years ago. They are a thick set of limestones, shales and sandstones typical of Paleozoic sections all across the middle of North America, with beds of rock salt as well.
The western section begins with Cretaceous rocks (green), some 140 to 80 million years old. They consist of sandstone, limestone and chalk. Younger rocks of Tertiary age (red-brown) represent a huge blanket of coarse sediment washing down from the rising Rocky Mountains, punctuated by beds of widespread volcanic ash. This wedge of sedimentary rocks was subsequently eroded in the last few million years; these sediments are shown in yellow. The light tan areas represent large fields of sand dunes that are grass-covered and inactive today. In the northeast, continental glaciers left behind thick deposits of gravel and sediment that they carried down from the north; the dashed line represent the glacier's limit.
Every part of Kansas is full of fossils. It's a great place to learn geology. The GeoKansas site of the Kansas Geological Survey has excellent resources for more detail, photos and destination notes.
I have made a version of this map (1200x1250 pixels, 360 KB) that includes the key to the rock units and a profile across the state.
More Kansas resources on About.com:
Kansas Maps
Kansas Geography, State Symbols & Facts
Kansas Campgrounds
Kansas Archaeology
