THE HERMOSA CLIFFS OF SW COLORADO: AN ANALYSIS OF MASS WASTING MECHANISMS AND HAZARDS

In the US, landslides cause over $1 billion in damage and over 25 deaths each year. Landslide hazards in a particular region require careful mapping, attention to the local geology, hydrology, and topography, and analysis of past landslide activity. Here we present progress on such an effort for an iconic stretch of cliffs located in southwestern Colorado. Along the “Million Dollar Highway” north of Durango, CO, is an imposing 15-mile, ~2000-ft high escarpment known as the Hermosa Cliffs. Although 1:24,000 Geologic mapping from 2004 identified deposits from 12 large landslides along the foot of the Hermosa Cliffs, development has progressed into and around those deposits. Fire-related debris flows coming off the Hermosa Cliffs in 2018 caused millions of dollars in property damage. And as development expands into the region on and near these large slide deposits, it is important to have a clear understanding of the specific hillslope hazards presented by the escarpment.

We used a combination of fieldwork and analysis of new USGS LiDAR data for the region (including surface roughness and slope), to re-map all quaternary deposits along the foot of the cliffs as a 1:10,000-scale ‘strip’ map. Relative ages of deposits were determined through analysis of cross-cutting relationships, surface roughness, and soil development. As part of that mapping, we distinguish among six types of deposits and their associated processes 1) larger, deep-seated landslides that appear to be generally older, 2) larger, more fluid landslides 3) rock avalanches of various age, 4) colluvial aprons below steep cliff faces, 5) alluvial fans constructed by debris flow and frequent channel avulsion, and 6) young, smaller slumps from the material on top of the cliffs. Together, these processes present a range of hazards to development below the cliffs. Further work will be required to determine whether any of the landslide deposits have been reactivated in recent centuries, but the mapping presented has already helped to refine the specific spatial patterns of the aforementioned hillslope hazards.