Within a small but beautiful Trustees of Reservations park called Agassiz Rock are two separate rocks of interest. Known simplistically as Little and Big Agassiz rocks, they are easily found on the loop trail.
From northbound Route 128 take exit 15 Manchester and turn left on School St. In half a mile or so, you will see a prominent gravel turnoff on the right with even more additional parking available on the left side of the street. This parking area will be marked with a Trustees park sign saying Agassiz Rock. Take the approach trail, and, after a short while, take one fork or the other for Little or Big Agassiz Rocks.