ON THE ROCKS Presents

Late Cenozoic Geology of the Upper Arkansas River Valley: 1.5 Million Years of Tectonics, the Central Colorado Icefield, and Formation of the Arkansas River

Friday June 26, 2026

Trip Leader: Cal Ruleman, USGS, Geosciences and Environmental Change Science Center

Trip Coordinator: Ralf Topper

Trip Description

Over the last 100 years, the elevation, neotectonics, and Pleistocene glacial extent of the central Colorado Rocky Mountains (CRM) have been debated amongst highly reputable geologists.  Recent geomorphic investigations aided by Quaternary geochronologic and thermochronologic tool development, have better constrained our understanding of Rocky Mountain landscape evolution resulting from Neogene tectonomagmatism and Pleistocene glacial and glaciofluvial processes.   Elevated mountain blocks and deep, narrow canyons host abundant clues and geologic constraints on the Late Cenozoic evolution of the Rocky Mountains and Intermountain West.  To highlight some of the new USGS geologic mapping and understanding, Cal Ruleman (U.S. Geological Survey) will lead a one-day field trip on Friday June 26, 2026.  We will visit several key localities along the Upper Arkansas River corridor, beginning at Fremont Pass and ending just north of Salida, highlighting classic geologic sites enhanced by new chronologic data.  We will discuss themes related to the cessation of the Laramide orogeny at ~43 Ma, initiation of Oligocene to Pleistocene tectonomagmatism, superseded by Pleistocene deep-canyon incision associated with the most recent Ice Age (~1.5 Ma-13.5 ka).  This trip will focus on the geologic history of the last 1.5 million years, the regional geomorphic impact of the Central Colorado Icefield, and the formation of the Arkansas River, linking geologic constraints extending from Leadville, Colorado to Little Rock, Arkansas.

Details

This one-day field trip will start at 9am on Fremont Pass and end near Salida around 4:30pm. Participants leaving from Denver can meet at the Wooly Mammoth RTD parking lot directly off of I-70 in Golden at 7:30am for carpooling options. All field trip participants will meet at 9am in the parking lot across from the Climax Mine entrance on top of Fremont Pass. All stops will be along the roadway so minimal walking is required. Participants are expected to bring their own lunch and water, closed toed shoes or boots, hat and sunscreen, and suitable clothing for unexpected changes in weather as well as elevation differences. Limited beverages will be provided at our lunch stop. Additional details will follow for those who sign up.This one-day field trip will start at 9am on Fremont Pass and end near Salida around 4:30pm. Participants leaving from Denver can meet at the Wooly Mammoth RTD parking lot directly off of I-70 in Golden at 7:30am for carpooling options. All field trip participants will meet at 9am in the parking lot across from the Climax Mine entrance on top of Fremont Pass. All stops will be along the roadway so minimal walking is required. Participants are expected to bring their own lunch and water, closed toed shoes or boots, hat and sunscreen, and suitable clothing for unexpected changes in weather as well as elevation differences. Limited beverages will be provided at our lunch stop. Additional details will follow for those who sign up.

Cost

• $55 for members
• $75 for non-members. (not a member? click here to join)
• Are you a student? email staff@rmag.org

Registration 

Registration Closes June 15, 2026. 

Refunds for the June 26th, trip are only available until June 12, 2026

If you are unable to attend, your registration for all RMAG events are transferable. RMAG members may transfer their registration to another RMAG member, and non-members can transfer their registrations to whomever they wish. Should an RMAG member wish to transfer their registration to a non-member, the non-member would need to pay the balance between the member and non-member price.

Waivers

All registrants must fill out the Waiver and Release of Liability and Medical Emergency Form. Those who fail to complete these form will not be permitted to attend the field trip.