Big mound next to Miscoe Brook in "Kittyville" Grafton

Was driving Rt 140 S. in Grafton, on my way back from exploring near Hassanamessit. Saw this in the woods, but big enough to see from the road:

I did not like where I was parked and left without getting closer. Possibly this is part of the millworks in this spot. Just about where the dot on the second "i" of the word "Kittyville" here: