Editing San Luis Valley (section)

====Laramide orogeny====
Building of the [[Rocky Mountains]] by folding during the [[Laramide orogeny]] 80 to 55&nbsp;million years ago created a highland, the San Luis-Brazos uplift, in the area of the San Luis Valley. This process over 30&nbsp;million years resulted in an elevated and highly eroded [[peneplain]] where the San Luis Valley is today. There was a basin in its western portion which may have drained west through the [[sag (geology)|course historically followed]] by the [[San Juan River (Colorado River)|San Juan River]]. Sedimentary deposits formed in that basin during [[Eocene]] times remain in place beneath the western portion of the San Luis Valley.<ref name=Trip2022/> During the [[Pleistocene]], parts of the valley were occupied by [[Lake Alamosa]].<ref>Machette, M.N., Coates, M-M., and Johnson, M.L., 2007, 2007 Rocky Mountain Section Friends of the Pleistocene Field Trip—Quaternary geology of the San Luis Basin of Colorado and New Mexico, September 7–9, 2007: U.S. Geological Survey Open-File Report 2007–1193, 197 p.</ref>