Like the Catskills as a whole, a dissected plateau, Halcott was formed not through the upthrust of rock layers but by the gradual erosion of stream valleys in an uplifted region about 350 mya. Its rock layers and bedrock are primarily Devonian and Silurian shale and sandstone. Later glaciation, primarily the Illinois and Wisconsin periods, shaped the slopes of the mountain, smoothing its slopes on the west when the ice was covering it and steepening the east slopes through erosion when the meltwater burst through Deep Notch.