LATE PLEISTOCENE AND HOLOCENE DEBRIS AVALANCHES RECORDED BY PROFILE LAKE, FRANCONIA NOTCH, WHITE MOUNTAINS, NEW HAMPSHIRE

Debris avalanching events are relatively common during historic time in the White Mountains, however, little is known about their frequency through the Holocene. Profile Lake provides an opportunity to establish a chronology of prehistoric landslide events. The lake is situated between two historic slide zones along the steep east wall of a classic U-shaped glacial valley and contains a complete Holocene record. One sediment core recovered from the middle of the lake is dated 10,480 +/- 170 14C yrs BP (GX-26620) at a depth of 6.04-6.14 m, where gray inorganic silt abruptly yields to weak gyttja above. Eight other sediment cores have been recovered in a transect along the length of the lake. Occasional lenses of coarse sand, which we suggest represent landslide events, periodically punctuate the sediment cores from the southern end of the lake. Documented accounts of eight local landslide events over the past 200 years provide an historic analog for some of these coarse lenses near the core tops. The most significant of these historic slides occurred on 07/23/47, 06/24/48, and 10/24/59, covering old Rt.3 with up to 8 m of debris and extending into the lake. Some of these landslides eroded the valley wall to bedrock, leaving scars that remain only partially vegetated today. All three major historic events were triggered by prolonged intense rainfall. LOI and magnetic susceptibility have proven useful in distinguishing less significant prehistoric landslide lenses in the sediment cores. Preliminary work suggests that slides appear to be clustered into three zones of higher activity separated by two periods of relative quiescence. The early and late Holocene appear to record the most frequent and largest events. Actual event ages await 210Pb dating of the historic parts of the cores and further 14C dating of coarse layers in the prehistoric parts of the sediment cores.