The Slumgullion landslide, in southwestern Colorado, is a complex phenomenon consisting of an active landslide currently moving on the upper-middle part of an older, larger, and inactive landslide. It is bounded for most of its length by flank ridges that are linear ridges of landslide material that have formed during movement along the lateral boundaries of the landslide. Flank ridges formed by deformation of landslide material, as well as formed by deposition of landslide material onto the adjacent ground surface, were recognized on the inactive Slumgullion landslide. Flank ridges are the best preserved features in the deposits of the inactive Slumgullion landslide. They extend continuously for several hundred meters, following the landslide boundaries. The several generations of flank ridges were mapped on aerial photographs and later field checked; observation of their main geomorphic, pedologic, and sedimentologic characters helped in reconstructing the chronology of their formation. An attempt was also made to correlate the identified sets of flank ridges with the known history of movement of the Slumgullion landslide. Maps and profiles were used to estimate previous width and thickness of the landslide at the time of formation of the identified sets of flank ridges. Comparison of these values with the present width and thickness of the Slumgullion landslide showed a thinning and narrowing trend, which will eventually result in inactivity of the presently active part of the landslide, unless a new amount of material is available from the source area.