Cordilleran Section - 99th Annual (April 1–3, 2003)

Paper No. 2
Presentation Time: 8:30 AM-5:30 PM


LETCHER, Alice J., Geology, Whitman College, 345 Boyer, Walla Walla, WA 99362 and CARSON, Robert J., Geology, Whitman College, Walla Walla, WA 99362,

Approximately 20 km2 of Mount Harrison and nearby Connor Ridge are scarred by about a dozen landslides. The slides are characterized by steep concave scarps, hummocky ground, springs, local block fields, and, on Mount Harrison, small ponds. The headwall scarps are mostly between the elevations of 2350 and 2550 m, with debris toes extending as far as 2 km downslope. The landslides, most of which have southerly aspects, are probably late Pinedale in age. Minor areas of reactivated mass wasting on three of the landslides consist of small slumps and earthflows, and have occurred as recently as a few years ago. Connor Ridge is the topographic expression of Big Bertha mantled gneiss dome. The landslides in this area occur on dipslopes of about 25°. Failure occurred within a schist along the contact of the Archean Green Creek Complex and the Proterozoic Elba Quartzite. The muscovite-biotite schist is a few to tens of meters thick. Major fractures in Big Bertha Dome may have influenced where specific landslides occurred. The landslide complex on the south side of Mount Harrison has headwall scarps originating near the probable lower limit of Pinedale glaciation. Multiple landslide lobes extend down drainages. Part of the mass wasting debris is likely Bull Lake drift. There is no evidence of structural control of the Mount Harrison landslide complex.