In northern Wisconsin, for example, a property might be entirely buried by glacial deposits. The nearest information about bedrock might be an outcrop a mile away on one side and a well a mile away on the other. If the bedrock is the same in both places we are probably safe in assuming it passes under the property as well. If the bedrock is different we can assume there's a contact someplace between the two sites, but there's no way of pinning its location down exactly. These maps are sold by the U.S. Geological Survey and many private vendors.
Indexes are available for each state.
Every state has a geological survey that produces geologic maps. In addition, the U.S. Geological Survey publishes geologic maps, and also produces indexes that include geologic maps published elsewhere, for example, in research journals or graduate theses. Map coverage ranges from very detailed to very sketchy. Much of the U.S., especially agricultural areas, is covered by soils maps . These are often drawn onto aerial photographs so that tiny details are visible.