#2. A Great Rift Runs Through It: River and Rock Overlook

Water has been an immensely powerful force in sculpting Turners Falls and the Connecticut River Valley ever since Pangea’s breakup began forming our rift valley 245 million years ago. Streams and rivers moved down the ancient mountains, sometimes at tremendous speed, but always, no matter their speed, wearing down the rock and carrying the resulting sediments into the valley. The continuous process of erosion and deposition is still forming our landscape.

THINGS TO SEE

The remains of an early Jurassic landscape. What you see across the river are rocks that were part of a landscape last visible about 200 million years ago, in the early Jurassic period–the days of the dinosaurs! Gradually buried by thousands of feet of sediment, that landscape lay invisible until water and other forces of erosion (wind, frost and thaw, cracking from the roots of plants) began uncovering it some 60 million years ago.

Introducing our village rocks. On this tour, we’ll see three rock types resulting from the tremendous forces that broke up Pangaea:

1. the Turners Falls sandstone or formation,
2. the Deerfield basalt, and
3. the Sugarloaf arkose. Across the river at Stop 2, you can see two of these rock types. (You’ll see the third, Sugarloaf arkose, at Stop 4.)

First, exposed along the northeast bank is the Turners Falls formation–a sequence of red sandstones, dark shales, and gray conglomerates–tilting or dipping southeast towards Boston. You can see the dip of the Turners Falls formation easily in the rocks exposed at the base of the bridge. Second, look for the dark, massive Deerfield basalt forming the ridge (called Canada Hill) that rises impressively on the northwest side of the river.

Once there was a lake. The world-famous Turners Falls formation, measuring about 750 feet thick, is the geological remains of a vast Jurassic shoreline, lake, and mudflats. Within about four blocks of the Discovery Center, at least four ancient Turners Falls formation lake beds dip to the southeast. Over many years, these ancient lake beds have been transformed into soft shale; one of the beds contains exquisite fish fossils. (Remember: Unless you have a permit, it’s illegal to collect rocks or fossils on public lands.)

THINGS TO KNOW

Reading the valley. The dips in the rocks help us “read” the valley. Originally, sediments were laid down flat (in horizontal beds) as the rift valley was forming. But as the sediments hardened to rock and the rift valley continued to form, the beds were tilted to the southeast as they sank. Because of this dipping, the beds of Turners Falls are stacked like the pages of a book, binding down, tilted on edge: Page 1, the oldest bed, lies to the northwest. Later, younger pages of our geological history book lie to the southeast.

Hot rock. Because the Earth’s crust is thinner under a rift valley than in foothills or mountains, hot, melted rock, called magma, comes closer to the surface. Here in our valley, this magma erupted over time through cracks in the Earth’s surface and flowed as lava across the valley floor. The lava then cooled and hardened into what we call the Deerfield basalt. Today the basalt, tilted up onto its edge, forms the north-south trending ridge, called Canada Hill or Rocky Mountain in Greenfield and the Pocumtuck Range in Deerfield. Poet’s Seat Tower rests on the Deerfield basalt.

Where are the falls? The Great Falls of the Connecticut River has been a gathering place for area residents for over 10,000 years. Native Americans and, later, colonists regarded the falls as one of the region’s premier fishing grounds. New visitors to Turners Falls wonder where to go to see these amazing falls. Stop 2, the River and Rock Overlook, would be an excellent place to view the falls, except for one thing: In 1798, the Great Falls was dammed to construct a navigational canal. The canal and nine locks allowed riverboats to travel from Long Island Sound up the Connecticut River to Vermont.

In the 1860s, the industrialists who founded Turners Falls rebuilt the canal and dam to provide power for their factories. Today, the dam and its canal generate electricity for everyone in the valley. Were the Great Falls not dammed, we would see a waterfall drop of about 40 to 50 feet – a reminder that water is still cutting and breaking down rock as it has for hundreds of millions of years.