Northwestern Pennsylvania was glaciated by the Grand River sublobe of the Erie Lobe. Glacial advances occurred at least three times during the pre-Illinoian (Slippery Rock, Mapledale, and Keefus), once during the Illinoian (Titusville), and four Late Wisconsinan (Kent, Lavery, Hiram, and Ashtabula) tills have been identified. While the area was studied for over 50 years by George White and associates, there are numerous details that remain unknown. The Titusville Till, which comprises the bulk of the glacial sediment, contains up to five separate sheets separated by sand and gravel. The origin of the numerous sheets is still not clear. The Kent glaciation resulted in extensive deposition in proglacial lakes and caused numerous local drainage diversions. Interpretations of the surficial geology around Conneaut Lake have changed a couple of times. Originally interpreted to be formed by a lobe of Hiram ice, it was later determined to be part of a widespread area of Lavery ice. Recent work supports the original Hiram interpretation. Since glaciation, streams in northwestern Pennsylvania have incised into the glacial sediments and have developed fine-grained floodplains within glacially scoured valleys. Lake sediments and alluvial stratigraphy suggests that general climate amelioration during the Holocene epoch was interrupted by episodes of landscape instability. Deforestation by European settlers is the most recent event appearing in the stratigraphic record and resulted in deposition of as much as 2 m of post-settlement alluvium.