In the Seneca Lake region, both the Ledyard and Wanakah members of the Middle Devonian Ludlowville formation have a black shale facies. This grades westward to a gray shale facies and eastward to a gray shale and siltstone facies. The black shale facies represents an anoxic basin of deeper water than the shallower water gray shale facies to the east and west. The axis of this basin trends northeast-southwest.

The Ludlowville formation from Lake Erie to the Genesee Valley has many thin argillaceous limestone beds, 1 to 4 in. (3 to 10 cm), that are useful for detailed correlation. Because these thin beds can be traced as far as 43 mi (70 km), this part of the outcrop belt probably parallels an ancient east-west shoreline. Between Genesee Valley and Seneca Lake, the inferred shoreline turns to a northeast direction parallel to the axis of the basin, and bedrock exposures display a barren, black shale facies. In these exposures the thin beds disappear. From Seneca Lake to Owasco Lake, the deeper water, black shale facies gives way to shallower water, gray shale facies with thin traceable beds. Because the thin beds disappear in the basin exposures, it becomes difficult to correlate the detaile stratigraphy of the western beds across the basin with the eastern beds. Some correlations have been made using ammonoids. The inferred northeast-trending shoreline may have circumscribed the northern end of this basin, and then turned southward to become part of the southwest-trending shoreline in eastern New York.

This basin first appeared during the deposition of the Early Devonian Helderberg Group and persisted during the deposition of the Middle Devonian Onondaga Limestone and Hamilton Group. Throughout this period, the basin axis shifted from eastern to western New York. This basin has already proven to be of economic importance as a gas producer from the Onondaga reefs that occur on its margin, and may provide other areas of economic importance.

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