The shallow Wellesley Intercept Tunnel passes N77W through a 1,576 foot-long section of bedrock in a great bend of the Charles River west of the Boston Basin in northeastern Dedham, Massachusetts. This MWRA sewer tunnel section which lies southeast of the east end of the Metro West water tunnel was mapped in great detail. The tunnel lies in a Late Proterozoic batholithic complex formed of an interfingering contact zone of the Dedham Granodiortie and its border phase, the Westwood Granite, invading Hornblende Diorite. This is cut by pre-Late Ordovician greenstone dikes and Early Jurassic diabase dikes.

The rock is cut by numerous faults with a spacing of 10 to 15 feet. These form several fault sets, and the dikes and interfingering of the rock appear controlled by earlier fault sets. The area is shown to lie in a broad northwest-trending fault zone, just south of an east-northeast trending border fault of the Boston Basin, yet relatively few faults of these trends are found. The most common faults trend north-northeast and northeast. The youngest are north-trending, near vertical faults intruded by the diabase and later locally reactivated. These form the most open zones; one leaked oil. Highly varied and closely spaced joint sets are ubiquitous and related to faulting, except for columnar joints in the diabase. The combination of joints and faults cut the rock into blocks about one foot across. Fortunately, most fractures are at a moderate to high-angle to the tunnel alignment. If more fractures were aligned with the tunnel there may have been severe problems. The most serious problem was high water inflow along the bedrock-Pleistocene interface at the southeast end.