THE 2011 KEENE VALLEY LANDSLIDE: A COMPLEX, DEEP, ROTATIONAL FAILURE IN THE ADIRONDACK MOUNTAINS OF NEW YORK STATE

On May fourth, 2011 the largest landslide in New York State’s history occurred less than one mile northwest of the Hamlet of Keene Valley on the side of Porter Mountain in the high peaks region of the Adirondack Mountains in Essex County. Approximately 82 acres of land area and five homes were impacted, one of which is totally destroyed.

The spectacle of the high peaks in the Adirondack Mountains largely is largely a result of the erosional resistance of the Precambrian Anorthosite bodies that comprises the bedrock. These mountain peaks and adjacent valleys have experienced multiple glaciations and thin glacial soils less than a few meters thick were previously thought to cover the mountain side as discontinuous patches resulting from the Adirondack Lobe of the Laurentide Ice Sheet during the Late Wisconsin.

The landslide appears to have been triggered by excessive groundwater originating from melting of extremely heavy snow pack combined with excessive April precipitation. The final tipping point came from heavy rains that spanned the last week of April into the first week of May. The excessive groundwater combined with complex glacial soils consisting of interbedded glacial lacustrine clay and silt deposits in excess of 25 meters thick draped on steeply dipping buried bedrock surfaces at high elevation. Glacial deposits of this thickness and composition were previously unmapped in this region.

The New York State Geological Survey (NYSGS) has monitored movement, mapped the landslide extent, conducted geophysical surveys and exploration drilling to characterize the geology and evaluate additional geologic hazards. While far from complete the rapid response and near constant monitoring of the still active landslide suggests a continued threat is very possible. Monitoring during large scale temporal events such as Hurricane Irene further illustrate that a complex groundwater system controlled by the complex geologic framework is present and driving the continued slope-failure. Characterization efforts combined with additional field investigations and new data in the region indicates that a continued risk of large landslides in the region is a very real threat to citizens, municipalities and infrastructure.