Abstract This road log runs from Montrose, Colorado, south along U.S. Highway 550 to Durango, Colorado, a distance of 110 mi (fig. 1). The route of the log crosses the San Juan Mountains along their western flank, crossing successively the Uncompahgre uplift, Sneffles horst, Silverton caldera, and the Grenadier highland (horst?) (fig. 2). Suggested stops along the way emphasize these geologic structures and the nature of the sedimentation and volcanism along the margins of the structures~ Secondary emphasis is on geologic hazards, especially snow avalanches.