The entrance to the park is on House Rock Road. The park has a grassy field with a basketball court and a swing set. When you walk through the gate, the rock is back to the left. You will see it when you get about halfway across the field. It's a massive, single piece erratic deposited by a receding glacier and is estimated at 3,500 tons (about 7 million pounds). Visit Weymouth’s House Rock for more detail on House Rock including references to "A resemblance to Queen Victoria ... a resemblance to the Great Sphinx of Egypt."
I have hidden this cache well due to the nighttime antics of teens without a cause! Please take care in re-hiding.
*Congratulations to Joe1407on the hunt for FTF!*