LANDSLIDES IN TULLY VALLEY, NEW YORK: PRESENT CONDITIONS AND HISTORIC ASSESSMENT USING DENDROCHRONOLOGY

The Tully Valley, near Syracuse, New York is a 6-mile-long glacial trough located in the eastern Finger Lakes region of the Allegheny Plateau. Onondaga Creek flows north through the valley, and bedrock is primarily shale, overlain by more than 400 feet of sand, gravel, and lacustrine deposits below the Tully Valley floor. Tully Valley has a landslide history dating back to 9,870 14C yr B.P., and in 1993, the largest landslide in the state since the early 1900's occurred on the west wall of the Valley. Presently, two slow-moving earth slides are located in tributary side valleys to the Tully Valley. New work with precipitation, ground water, and movement measurements, field mapping, and dendrochronology provides a data set for present analysis and historic assessment of the Tully Valley landslides.

The Rattlesnake Gulf landslide is composed of approximately 400 feet of rotated laminated silt and clay deposits dipping 14°-55°SW. The Rainbow Creek landslide is composed of a 10 foot thick layer of laminated clay overlain by a 40-foot-thick layer of rotated, interbedded sand, silt, and gravel dipping 12°-34°SE. Blocks of unconsolidated material in the Rattlesnake Gulf and Rainbow Creek landslides are moving at approximately 10 feet/year and 1.5 feet/year respectively. Movement is triggered by stream erosion of the toe of each slide and(or) the movement of infiltrating precipitation and runoff to the shallow ground water system. The Rattlesnake Gulf landslide has a terraced appearance because sediment is sliding as a plastic mass, whereas the Rainbow Creek landslide appears to be moving as several large blocks.

Dendrochronology can be used to determine historic movement in these landslides because tree rings grow eccentrically when earth movement causes a tree to tilt. At Rattlesnake Gulf, initial tree ring analyses on Quercus alba and Betula lutea indicate significant movement in 1970. At Rainbow Creek, initial analyses of Tsuga canadenses indicate movement occurred in 1983 and 1989. For each of these years, the first three to six months have precipitation deficits of at least 3 inches, which are followed by several months of excess precipitation of 4 to 7 inches. These initial results suggest relatively dry periods followed by excessively wet periods trigger significant landslide movement in the Tully Valley region.