Geologically Griswold is very interesting and has been studied for years by geologists from all over New England. There are no natural ponds in Griswold. The larger part of its surface is drained by the Pachaug River.

The distance from the source of the Pachaug to its mouth in a straight line does not much exceed 9 miles, yet it winds with such surprising curves, and so tortuous is its natural course through the swamps and mud flats between the hill, that the waters of its source travelled is 44 miles before they reach the mouth.

The Quinebaug flows through three ranges of hills. Stone Hill in the northeast corner of town is 580 feet above sea level; while on the south the summits of Rixtown and Ray Mountains stand 500 and 560 feet above sea level respectively. The sides of these hills, at least at the 200 foot level, show unmistakable signs of the long continued swirl and flow of water proving that Griswold basin at a distant period belonged not merely to the valley but to the bed of the once grand Quinebaug.

Evidence of glacial activity abound. Not only is the soil strewn with glacial boulders,some of which are remarkable, but proof is found in deposits of till, the peculiar shaped hills, the pit-like kettle holes, the beds of clay, the underlying hardpan and the polished surface of the ledges.