Paleontology and geology
The most commonly encountered Paleozoic rocks in Idaho are Early Carboniferous (Mississippian) marine sediments consisting mostly of carbonates, but also some sandstones and shales. These rocks (included as undifferentiated on this map) are exposed in the central and southern parts of the state. The most common marine fossils are horn and colonial corals, brachiopods, trace fossils, algae, bryozoans, crinoids, gastropods, pelecypods, cephalopods, but fossils of trilobites and fish have also been reported. Poorly preserved plants and even some thin coal layers can also be found.
The Late Carboniferous (Pennsylvanian) is represented by marine sediments in the central and southeastern regions of the state (included as undifferentiated on this map). The variety and quantity of fossils is much poorer than in the Early Carboniferous (Mississippian), but fossil algae, fusilinids and other foraminiferan, bryozoa, brachiopods, crinoids, corals, and molluscs have been found in these sediments. The rock surfaces indicate that marine sedimentation was interrupted by periods of uplift and erosion; however, terrestrial deposits have not been found.