The uplands that comprise the Hoosic watershed are part of the Appalachians, a once-spectacular mountain chain that formed millions of years ago when continental plates collided. Among other things, this event (the Taconic Orogeny) caused offshore islands to be pushed west into what is now New England, which is why local mountains contain layers of rock formed under the ocean, such as quartzite, schist, and limestone.
The Green River is sometimes greenish because finely powdered rock left over from glacial rock grinding is still washing down from the hills.
The Hoosic cuts deeply through these old uplands, and it is one of the few rivers in this region that did not succumb to the north-south gouging of the glaciers 10,000 years ago. As the glacier melted, the Hoosic returned to its pre-glacial riverbed, to flow northwest and carry off water from glacial Lake Bascom. Traces of that lake are still visible as gravel and sand beaches, now 1,300 feet above the river valley in northwestern Massachusetts and southern Vermont.