The paper is based on information obtained by the writers and other members of the United States Geological Survey between 1927 and 1932. The stratigraphy of central Colorado is briefly described. Unconformities occur at the base of the Cambrian, within the Ordovician, and beneath the Devonian, Mississippian, and Pennsylvanian deposits. The thickening and thinning of beds beneath the unconformities and overlaps are described and illustrated by sections. Apparently the Harding and Fremont formations of the Ordovician were originally much more widely distributed than at present. Long intervals of erosion preceded the deposition of the Devonian and the Pennsylvanian deposits, during which large amounts of earlier sediments were removed.

The erosion indicated by the widespread unconformities is comparatively small throughout much of the region, but in a narrow northerly trending zone on the western side of the Gore Mountains the erosion during each interdepositional period was so severe that nearly all the material just deposited was removed. The irregular, interrupted succession of overlap in this zone gives field evidence for the persistence of the Front Range Highland.