The local geology is noted by the presence of lowland stream valleys along Main Street with low rounded bedrock and glacial hills rising up to 100 feet above the surrounding area. The glacial deposits in the area have modified the landforms so that the present surface is more smoothly contoured than the underlying bedrock. Kelly Hill is composed primarily of granitic rock, with lesser amounts of the metamorphic rock, gneiss and schist (USGS, "Surficial Geology of the Wilmington Quadrangle, Massachusetts", 1959). Several granite outcrops were noted along the south side of Kelly Hill during field inspections. Overlying the bedrock on Kelly Hill is a thin layer of loose sand and gravel up to 15' thick. These deposits are often referred to as ablation till and glacial outwash, which occurred from within the glacial ice. This material was deposited during the glacial melting and is generally very sandy, bouldery, and loosely compacted. The glacial outwash deposits were found to be the thickest in the stream valleys along Main Street. Environmental assessment reports completed for the former Wilmington Ford site (275 Main Street) and the JJT site (315-319 Main Street) document the depth to bedrock to be approximately forty feet below the ground surface. Overlying bedrock in the lowland areas is generally a 2-10' thick layer of basal till. The basal till or "lower till" is a highly compacted sand, gravel and clay mixture which was formed by the immense weight of the overlying glacial ice. The hydraulic conductivity of basal till is very low, which is in sharp contrast to the overlying loose sand and gravel which is highly water transmissive.

All commercial, industrial and residential properties in the study area have had private septic systems to date. The Town of Wilmington is presently constructing a municipal sewer which will be available in the near future for all commercial and industrial activity located along Main Street.