The Ziegler Reservoir fossil site near Snowmass Village, Colorado (USA) provides a rare opportunity to examine environmental conditions in the Rocky Mountains during marine isotope stage (MIS) 4 (71–57 ka). Although recognized as a global-scale cold event, MIS 4 is typically absent from Rocky Mountain glacial chronologies because the geologic evidence was either covered or destroyed during the subsequent, and larger, MIS 2 (Pinedale; 29–14 ka) glaciation. The Ziegler Reservoir site is perched on a ridge above Pinedale glacial limits, which allowed for the preservation of a long-lived sequence of predominantly eolian sediments that spans from late MIS 6 (ca. 140 ka) through early MIS 3 (ca. 55 ka). Sediments dating to MIS 4 exhibit a significant increase in clay-sized particles, which suggests the source areas were enriched with fine-grained material at that time. We hypothesize that this increase in clay content is the result of rock-flour production by nearby valley glaciers that were active in the Rocky Mountains during MIS 4. If correct, our study illustrates how recognizing indirect evidence of glacial activity can result in a more complete record of past climate conditions than what could be achieved by the study of glacial moraines alone.