AN OVERVIEW OF SOME LARGE LANDSLIDES IN PENNSYLVANIA

Recent landslide events in the western U.S. and slightly older large slide events in New York State have brought national attention to landslide hazard, and suggested a review of known large slides in Pennsylvania. Geologists have long known about the common occurrences of landslides in some parts of Pennsylvania. In the most slide-prone areas, such as the Greater Pittsburgh area, typical individual slides are smaller than 1000 ft, are developed in colluvial soils, move slowly, and primarily cause damage to property and infrastructure. Large slides seem to involve different conditions.

Some large landslides are documented in the literature, but availability of statewide lidar elevation data has allowed detection of some previously unrecognized features and improved our view of known ones.

A number of large bedrock slides occur in Devonian and Mississippian sandstones on dip slopes in the Ridge and Valley Province and Allegheny Front Section of the Appalachian Plateaus. The largest of these slides are 2000 ft or more in length, width or both, with one example exceeding 3500 ft. Local relief on the affected slopes is 500 to 1400 ft.

Slides involving glacial deposits in northeast and northwestern Pennsylvania are typically less than 1000 ft in maximum dimension, but we have some examples up to about 2000 ft. Slopes in glaciated areas are generally lower, with thick unconsolidated deposits largely restricted to lower slopes and valley floors. This limits the potential for large landslides in these settings.

Where ages can be determined or inferred, most large slides are old in human terms. Absolute age is not easy to determine for many of them. Braun et al,(1989) date the largest of the bedrock slides along the North Branch of the Susquehanna River as younger than 4000 years.

Only a few of the large slides are known to be active, with continued movement affecting highways. As is common in smaller bedrock slides, these may have been triggered or reactivated by construction activities removing support at the toe. Continual stream erosion at the toe is common among the active slides in glacial deposits.

Although any of these slides could become active if moisture or slope conditions change, it is unlikely that landslides of the magnitude and effect of the 2014 slides near Oso, Washington and Grand Mesa, Colorado will occur in Pennsylvania.