This sequence of sedimentary rocks, containing coarse gravel, sandstone, and clay-rich mudstone, bears witness to the uplift and erosion of the Rocky Mountains. The character of the sedimentary rock changes dramatically as you move across the Denver Basin. To clarify the sequence and name it, members of the Denver Basin project grouped the beds by age. The 900–1,800-foot sequence represents the first episode of uplift along the Front Range. It is called D1 and includes rocks known as the Denver Formation, Arapahoe Conglomerate, and part of the Dawson Arkose.

Abundant fossil leaves in this unit allow researchers to determine the characteristics of ancient climates and rainfall patterns. Additionally, much of Denver’s groundwater resources are contained in these rocks. By studying their composition and bedding character, resource managers can better determine how to best develop these finite water resources.