Near the top of the loop road at Hopkinton State Park

The usual advisory is to look to the right as you drive up the loop (clockwise). As you look into the woods that are uphill from a picnic area, there are some nice piles. Downhill to the left are some other impressive ones. FWIW I saw a couple new ones at the top of the hill on the north side:

Hopkinton State Park is pretty busy with structures.