Trash Paddler: Looking for Saddle Rock

While reading Robert M. Thorson's The Boatman (Henry David Thoreau's River Years) I came across the author's mention of a boulder historically used in measuring the river's depth. The closest thing Musketaquid had to a local stage gauge was a large boulder known as Saddle Rock, located at Heard's Bridge at the southern end of Sudbury Meadows.