The Keuka Lake region is part of the Allegheny Plateau, where shale and sandstone formed from mud and sand deposited on a shallow sea floor over millions of years and later hardened into rock through lithification. Fossils such as trilobites, brachiopods, and corals are found in area gorges and glens, and the surface rocks belong to the West River Group, consisting of sandstones, siltstones, and shales derived from erosion of the Acadian Mountains to the east, themselves formed by collision of the North American and Avalonian Plates. The region became dry land more than 300 million years ago in late Devonian times, after which uplift and erosion removed about one vertical mile from an original two-mile-thick stack of rock, with rivers including one in the Keuka valley cutting through the area. Beginning about two million years ago in the Pleistocene, repeated glacial ice sheets from the north scoured the landscape, deepened north-south valleys from V-shaped to U-shaped forms, and sculpted ridges into streamlined shapes; the last continental glacier was at least one mile thick. As that glacier melted and retreated, it paused about 17,000 years ago and formed the Valley Heads Moraine, which blocked southward drainage and left part of a barrier still visible in the hilly topography between Hammondsport and Bath. Around 15,000 years ago, water in the Keuka valley rose to about 1,000 feet above sea level, overflowed from the west branch into the east branch, and as water passed between the present day Keuka Lake State Park and Route 54A, the Bluff became an island for about 300 years while meltwater also cut the outlet toward Seneca Lake. Keuka Lake then gradually fell to about 714 feet above sea level. Erosion of softer shale beneath harder sandstone at the Bluff's crest allowed the Bluff to resist wearing away as ancient rivers flowed around it, helping create Keuka Lake's distinctive Y shape. Major inlets are at Hammondsport and Branchport, the outlet is at Penn Yan where gates regulate the lake level, and the lake drains to Seneca Lake and then northward through the Seneca-Oswego-Oneida Rivers Drainage Basin to the Atlantic Ocean by way of Lake Ontario and the St. Lawrence River. In the Town of Jerusalem, the Bluff and the Pinnacle are the highest points at about 1,400 feet above sea level.