The Eagle Basin occupies a part of the Maroon trough that began to develop in Early Pennsylvanian time. The early sediments are clastics, carbonates and evaporites. Coarse clastics influenced sedimentary types throughout Triassic and Jurassic deposition. In Cretaceous time black shale and marine sandstones were deposited, then lagoonal and paludal sediments including coal. At the end of Mesozoic time the Eagle Basin was uplifted, folded and faulted, and the Piceance and other basins were formed. All important Paleozoic, Cretaceous and Tertiary oil and gas fields are related to the local improvement of porosity and permeability by fracturing in the sediments encasing the reservoir.