This stone monument just off of outer Congress Street in Stroudwater Village marks the beginning of the Cumberland and Oxford Canal, a Maine Historic Civil Engineering Landmark.
Wikipedia sums up the canal nicely:
"The Cumberland and Oxford Canal was opened in 1832 to connect the largest lakes of southern Maine with the seaport of Portland, Maine. The canal followed the Presumpscot River from Sebago Lake through the towns of Standish, Windham, Gorham, and Westbrook. The Canal diverged from the river at Westbrook to reach the navigable Fore River estuary and Portland Harbor. The canal required 27 locks to reach Sebago Lake at an elevation of 267 feet (81 m) above sea level."
Unfortunately for the canal, a satisfactory railroad network was built not long after that rendered the canal obsolete.This photo shows the beginning of the canal, as it connects into the Fore River:
And here's the view in the opposite direction of some of the homes in Stroudwater Village (your monitor is working correctly, I just played around with colors on this photo):