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Publications - Occasional Papers

Landslides Near Aspen, Colorado

INSTAAR Occasional Paper 20

1976, 61 pp. 5 plates. (cost: $5)

Sixty-two landslides have been identified in a 68-square-mile area west of Aspen, Colorado, and mapped, primarily from air photographs. Their distribution is a clustered one which reflects the spatial control of landslide location.

Landslides are found on every major lithologic unit in the area and landslide characteristics do not differ significantly between lithologies. They are, however, located preferentially on east-facing slopes of north­trending valleys and on north-facing slopes of east-trending valleys. Nearly all landslides also involve a component vector of bedrock dip in the direction of slope. No landslides have starting zone slope angles of less than 16°.

Age determinations, based on vegetation and stratigraphy, suggest an age range of pre–Bull Lake to presently active, with most of the landslides considered to have occurred soon after 11,000 BP in association with conditions of the immediate postglacial time. A few slides have been initiated in the past 100 years.

The most critical parameters involved in landslide initiation are water and slope angle. Dry slopes and those at an angle of less than 12° are not expected to slide. Inactive old landslides and slopes of morainal deposits or Mancos Shale are exceptionally susceptible to movement. The margin of stability over the entire area is considered to be slim and caution is advised in land use, especially where a changed water use is involved.

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