MILLION CUBIC METER SCALE POST-GLACIAL ROCK BLOCK SLIDE ON CADILLAC MOUNTIAN, ACADIA NATIONAL PARK

Pleistocene glacial ice and meltwater erosion deepened a north-south col between Cadillac and Dorr Mountains by about 60+ meters. The lower part of the col is a 30+ meter high vertical cliff with talus apron. The col cuts the N15⁰ W, 10 – 20⁰ NE dipping sheeting in the Cadillac Mtn. granite, daylighting the sheeting dipping out of the west side of the col and into the east side of the col.

The slide is marked by a headwall fissure running 600 - 650 m parallel to, 120 m west of, and 60 - 65 m above the col floor. A lower intermediate fissure runs 400+ m parallel to, 80 m west of, and 50 m above the col floor. The upslope head of the slide is marked by a 5 – 8 m high scarp with a 6 - 8 m wide graben below it. The intermediate fissure is marked by a 5 – 6 m headwall scarp with a 10 -13 m wide graben. Other narrow fissures branch off the 2 primary fissures. The grabens have a 2 – 3 m high, discontinuous downslope wall and are floored by a jumble of blocks 1 – 5 m across. The slide is about 30 m thick, assuming the failure plane lies near the base of the 30 m high cliff in the col. The volume of the slide mass is on the order of 2.2 x 10⁶ m³ if the failure plane dips at an average of 15⁰ from head to toe of the mass. If the failure plane steps up the sheeting under the slide, the volume of the mass could be markedly reduced but still is probably on the order of 1.0 x 10⁶ m³.

The slide only moved at most 20 m or so horizontally and 10 m or so vertically before stopping, probably due to increased friction from fragmentation of the granite along the failure plane and/or striking the west edge of the floor of the col. A similar situation has been observed on Shickshinny Mtn. in PA, where a number of sandstone dipslope rock block slides have broken loose, opening up headwall grabens, but leaving the blocks hanging on the mountain side. The Shickshinny Mtn. slide blocks show opened up bedding planes and joints along even the downslope sides. Such opening of the joints is also observed in the Cadillac Mtn. slide mass on the downslope face in the col.

The slide most likely occurred immediately after deglaciation as support of the ice was removed and the slope was saturated with maximum pore water pressure in the sheeting joints. The failure probably occurred in 2 phases, with the lower 80 m wide block sliding out and then the upper 40 m wide block sliding into the lower block immediately there after.