The prevailing view in the academic community 50 years ago was that central and eastern Massachusetts consisted primarily of Paleozoic rock that had been complexly folded and was unfaulted except for a few Triassic offsets. Extensive, generally unpublished, quadrangle mapping and geophysical explorations by the USGS starting in the 1960s showed the rock to be generally older, intensely faulted, and with few folds other than drag along faults. However, the State Geologic Map of 1984 largely shows the former view, which also influenced exploration for the MetroWest Tunnel. Predicted tunnel conditions were presented in terms of large structural blocks instead of specific geologic features.

Most of the MetroWest Tunnel lies in the outer, northwestern portions of the Late Proterozoic Milford Granite and Dedham Granodiorite, and is highly fractured with little evidence of folding. Relatively narrow fault-defined zones and several sets of faults have been mapped both in the tunnel and on the surface. Zones of water inflow and weak rock, several to a few hundred feet wide, are associated with faults and shear zones, especially at their intersection, and with adjacent hydrothermally altered rock. These data support the geology from quadrangle mapping and demonstrate that outcrop mapping by experienced field geologists can more accurately predict geologic conditions at tunnel depths. An exploration program combining the MetroWest map data, detailed surface mapping, and site-specific boring data can provide a more accurate and regionally specific model for predicting tunnel mining costs.