The oldest bedrock formations in the Mt. Holyoke quadrangle, Massachusetts, are feldspathic schists, tentatively correlated with Emerson's Amherst schist, and pegmatite lenses, perhaps of late Paleozoic age. Next in order of decreasing age are the Triassic Sugarloaf arkose and Longmeadow sandstone, with a 300- to 400-foot diabase flow between them. The Granby tuff, with associated lava, pipes, sills, and dikes of diabase was erupted in early Longmeadow time and records an unusual phase of Appalachian Triassic vulcanism.
The bedrock floor is extensively covered by unconsolidated Pleistocene and post-Pleistocene deposits. Only one till, presumably Wisconsin, is recognized. In the Connecticut Valley proper, lacustrine and outwash deposits of varved clay, silt, sand, and gravel are widespread. Recent flood-plain deposits consist of silt and sand.
Rural settlements and dairy farms derive nearly all the water from the top of the late Pleistocene varved clay, an ideal aquifer at shallow depth. The Triassic sedimentary rocks could supply copious amounts of water, and artesian conditions should exist over part of the area.