Franklin County Geology

Franklin County has magnificent landscape scenery: mountains, waterfalls, rushing rivers and meandering ones—such as New England's longest river—the Connecticut. We also have a great bunch of rocks, too, including famous ones with dinosaur footprints and something truly unique: Jurassic Armored Mud Balls! Greenfield Community College has the world's best examples of these delicate, photogenic, amusing and exceedingly rare "sedimentary structures." They are to be found in an N350 hallway display case as well as monumental samples along the GCC Geology Path.

Here is a very brief trip through the "deep time" geology story of Franklin County: In the Paleozoic Era (starting c. 500 million years ago) western Franklin County was the underwater edge of the North American continent. Crustal "plates" made up of continents and oceans came from afar and, through plate tectonics, collided to make the supercontinent of Pangea. Those collisions created schist and slate metamorphic rock out of the old ocean sediment. Old continental crust became the metamorphic rock gneiss.

The collisional stresses in several stages over several hundred million years created the Appalachian Mountains. Franklin County was "landlocked" in the middle of Pangea surrounded by the young Appalachians, now mostly eroded.

Supercontinents can't live forever. Heat builds up from below and the crust expands and cracks like a loaf of bread. The cracks are "rift valleys" and that is extremely important in the development of our unique geology. Plate tectonic processes move the up-arched continental pieces outward., eventually creating a space for the Atlantic Ocean and the continent shapes and positions we see today.

In the Mesozoic Era, Pangea split created a long faulted "rift valley" which looked much like the stark, dramatic landscape of Death Valley. The climate was tropical and semi-arid, being located in the middle of Pangea at roughly 15-20 degrees North Latitude, similar to central Mexico today. Streams washed great volumes of gravel, sand, and mud into the rift valley. Dinosaurs walk through the muddy sediment of lakes and streams. Sometimes if the conditions were just right, mud chunks from eroding stream banks rolled down-stream and became armored with pebbles. The fragile balls had to be quickly covered with more sand and gravel, and then buried to be preserved. Finally, 200 million years later, they were luckily discovered in a few quarried stone pieces from Turners Falls.

Because of the continental splitting, lava erupted along faults and "fissure flows" of basalt totaling over 150 feet flooded across the valley. Please note that we have lava flows, but no evidence of large volcanoes. As the lava cooled, it shrank and cracked. Those polygonal cracks form the spectacular columns, as the basalt is exposed by erosion. You can see some great examples at the top of the GCC Geology Path

More sedimentary layers covered the lava and continued fault movements (earthquakes) tilted the whole rift valley "sandwich" of sedimentary layers with a lava middle.

Skipping about 200 million years ahead to the late Cenozoic Era, glaciers advanced and melted. The last glacier melted out of Franklin County about 16,000 years ago. But, this melting glacier had a prominent glacial lake that followed the retreating ice front from central Connecticut and through Massachusetts and northward. This was glacial Lake Hitchcock which finally drained about 14,000 years ago. Lake Hitchcock accumulated a great quantity of sediment from the melting ice and inflowing rivers. Those deposits became a major shaping force of today's landscape and human history along the Connecticut River Valley! Lakebed mud (silt and clay) was a source for many Franklin County town's historic brick making companies and the flat stone-free lake bottom and river floodplains are the source of rich soils for agriculture. Waterfalls were created as the Connecticut River came back to its valley, carved a new path through the lake sediment, and found itself flowing over bedrock. Those waterfalls created the waterpower for historic industry, such as at Turners Falls, and now provide hydropower to the electric grid.

The Connecticut River once came to GCC! The early post-lake Connecticut meandered into the Greenfield Meadows and along College Drive leaving an old oxbow channel, now the curving wetland seen on the left as you enter the campus road. The flat parking lots and East Building sit on the old lake bed. The small campus ponds, by the way, are sections of a small stream that was altered during the early 1970's campus construction. They are not kettle holes from melted glacial ice.

Franklin County is a very special place. It is quite different from other areas of New England. With lava flows and abundant red sedimentary rock, the same type as Ayers Rock (Uluru) in Australia, as well as beautiful metamorphic deep-Earth rock, Franklin County is a geologic adventure in rocks and landscapes that all can learn from and enjoy. GCC is fortunate to have specimens to see and touch representing Franklin County's amazing geologic heritage!