North-Central Section - 39th Annual Meeting (May 19–20, 2005)

Paper No. 5
Presentation Time: 8:00 AM-12:00 PM


FLEEGER, Gary M., Pennsylvania Geol Survey, 3240 Schoolhouse Road, Middletown, PA 17057-3534,

An eroded lake bluff on the east side of Pymatuning Reservoir, Crawford County, PA, appears to be a key to several unresolved issues concerning the glacial history of northwestern PA. It also creates additional questions.

Over the years, the changing interpretations of the deposits in the Shenango River valley have caused considerable confusion. Shepps (1955) mapped a lobe of late Woodfordian Hiram Till down the valley, including the bluff location. White et al (1969) expanded the extent of middle Woodfordian Lavery Till (similar to Hiram) to include the Pymatuning site, but still included the Hiram lobe on their map. However, White (1982) indicated no Hiram in the valley, but further extended the Lavery boundary.

The bluff is about 20 feet high. The sediments are oxidized to the base. They are leached of matrix carbonates to about 4 feet. This depth of oxidation is unusually great, even for older tills. The top 14 feet of the bluff is massive, fine-grained, sparingly pebbly till. The lower 6 feet contain numerous deformed channel-form masses of sorted sediments, stingers of sorted sediments within the till, and vice versa.

The clay mineralogy and matrix carbonate content of the tills suggest a weathering break between 7 and 10 feet from the surface. This suggests that there are 2 tills, interpreted as Hiram over Lavery, present, and that a period of weathering occurred between the deposition of the 2 tills. An exposure of early Woodfordian Kent Till over Illinoian Titusville Till, about 1200' NE of the bluff, suggests that the Hiram and/or Lavery are confined to the valley, and that there may well have been a lobe of ice extending down the valley at least once.

The bluff is on the NW side of a peninsula into the lake. To the SE of the bluff are two islands. The surface of the peninsula and of Ackerman Island is irregular with closed depressions. Perhaps the peninsula and islands are the remnants of a moraine built along the margin of the ice lobe. Smaller exposures on Ackerman Island have a geology similar to the lower part of the peninsula bluff.

The sediments in the lower 6 feet of the bluff show evidence of significant subglacial shearing. The channel forms of sorted sediments are greatly deformed, including diapiric intrusions of till, and till stringers within. The super- and subjacent till contains numerous stringers of sorted sediments.