Geology of Our Watershed
The bedrock underpinning of the Nashua River watershed is made up of two types of rock: granite and some other igneous types of rock, and metamorphic rock, primarily schist and gneiss.
Over ten thousand years ago, during the Pleistocene Epoch, the Nashua River valley was carved by moving glacial ice that was over one mile deep. The Nashua River itself was once Glacial Lake Nashua, an enormous lake that extended from Boylston, Massachusetts north to Nashua, New Hampshire. At this time, it flowed southward through the Worcester area.
Bedrock and a thin layer of glacial till “hardpan” dominate the higher elevations of the watershed, especially to the west and northwest, where the main tributaries to the river rise (Quinapoxet, Stillwater, North Nashua, Squannacook, and Nissitissit Rivers). These rivers all flow from the northwest to the southeast. The rivers meet the Nashua River at sharp angles, turning to join the Nashua River to flow in a northeasterly direction (except where the Quinapoxet and Stillwater Rivers enter the artificially created Wachusett Reservoir). The flow of the tributaries is a piece of the evidence that the Nashua River used to flow south. The river’s course was reversed as the edge of the last ice age glacier melted away, leaving Glacial Lake Nashua to drain to the north.
The central part of the watershed is dominated by sand and gravel deposits created by the valley’s history of glaciations. These deposits are the groundwater source for many of the watershed’s municipal water supplies.
For a fascinating hike, take a walk at the Oak Hill Conservation Area in Harvard and Littleton, MA. There you can see Tophet Chasm, an ancient outflow of Glacial Lake Nashua. Lake water rushing out through a weak fault area in the hill, created a 120 foot waterfall and carved this deep chasm. The chasm is now dry, but still impressive. The area is the property of the Littleton Conservation Trust.