GEOLOGIC HISTORY OF THE CONNECTICUT RIVER VALLEY NEAR GREENFIELD, MA

BARTON COVE, MA, VIEW TOWARD TURNERS FALLS


For inquiries contact Richard D. Little Professor of Geology, Greenfield Community College, Greenfield, MA

GENERAL SETTING

Greenfield, bounded on the east by New England's largest river, the Connecticut, and on the west by the highlands of the Berkshire Hills, is one of the best places in the world to study geology. All three rock types (igneous, sedimentary, and metamorphic) are easily visible on our landscape and sand, gravel, and clay deposits from the last ice age are also abundant. Topographically, Greenfield's main residential and business area is about 250' above sea level, on the flat lake bottom plain of Lake Hitchcock, a glacial lake that drained about 14,000 years ago. To the west early Paleozoic Era schist rocks of the Berkshire Hills reach elevations of 1100', while several miles away at the east end of town a prominent, scenic ridge, site of Poets Seat Tower, rises 250' abruptly above the flat valley lowlands. It's composed of early Jurassic age lava, 194 million years old, that was tilted by faulting as the great supercontinent of Pangea broke up. The more gentle eastern side of the lava ridge slopes to the shoreline of the Connecticut River which has a scenic 40' waterfall (Turners Falls) just before entering Greenfield, and flows along a valley with abundant outcrops of red sandstone sedimentary layers, which, sometimes, display dinosaur footprints!.

PALEOZOIC ERA EVENTS

Things were quite different in early Paleozoic times. About 600 million years ago this region was south of the equator and under the sea at the tropical edge of North America, located in the old Atlantic (known as Iapetus Ocean, named for the father of Atlas). Over several hundred million years the processes of plate tectonics propelled several landmasses to collide with this edge of North America, making mountains with each impact. The first of these orogenies (mountain-building episodes) was the Taconic. It has been recently proposed that an impact with western South America (which was part of the great southern continent called Gondwana) pushed up these mountains 430 million years ago because similar rocks and fossils are found there. As with all mountains, erosion lowered them over 10's of millions of years, and eventually the warm, tropical seas covered the region once again.

An even bigger collision came next. About 400 million years ago the northwest Africa side of Gondwana hit North America creating the Acadian Orogeny which built the northern Appalachian Mountains. Greenfield and surroundings were now part of a major mountain range as high as the Himalayas. The effect of all this stress on the rock was to change these tropical marine sedimentary layers into metamorphic rocks: banded gneisses, shiny schists, and slates. Commonly parts or all of these layers melted and the magma cooled slowly into the igneous rock, granite.

As the Paleozoic Era ended, erosion had worn down these lofty peaks, but the ocean never again came to Western Massachusetts. We were in the middle of the great supercontinent known as Pangea.

MESOZOIC ERA

PANGEA - THE SUPERCONTINENT

While the bedrock basement of the Connecticut Valley is the Paleozoic metamorphic and igneous rocks described above, something very different is about to begin. The supercontinent of Pangea starts to split. Stretching stresses break the preexisting rocks creating faults that are much different from those due to compressional stresses of Paleozoic orogeny. These faults are "rifts" -- great "pull-aparts" that create prominent valleys (rift valleys or grabens) such as seen today in Death Valley and its relatives out West, as well as the famous east African valleys where our ancestors lived and evolved.

The present-day Atlantic Ocean begins to form as water spills into the largest of these cracks, and the Connecticut Valley develops along one of the adjacent rifts known as the Eastern Border Fault. It defines the eastern edge of the Connecticut Valley and is over 100 miles long, from Keene, NH to New Haven, CT and beyond.

Rivers washed great amounts of sediment into this valley. Great alluvial fans formed against the Eastern Border Fault margin as rivers deposited thick piles of gravel adjacent to the mountain front. Farther into the valley sand and mud were left. Many times lakes formed in the valley flats with sandy shorelines and muddy bottoms. The Connecticut Valley has as much as 30,000 feet of sediment preserved, but in the Franklin and Hampshire County area of the valley, the thickness is about 6,000 feet. But think of that -- over a mile of river and lake deposits, layer by layer, representing 10's of millions of years of history.

JURASSIC ANIMALS AND PLANTS (HYPOTHETICAL SCENE FROM THE CONNECTICUT RIVER VALLEY)

It's these wet valley flats that preserve the footprints of dinosaurs and other reptiles and amphibians. The most common trackmaker here was not the famous Tyrannosaurus or other huge dinosaurs (they came later in time), but (probably) Dilophosaurus, Coelophysis and relatives. It's hard to say for sure since bones, which would define the species of animal, have never been found with the prints. Dilophosaurus, by the way, was portrayed in the movie "Jurassic Park" as a poison-spitting dinosaur. These dinosaurs were about 20 feet long and stood about 6 feet high. They left prints ranging from 12 - 18 inches.

DINOSAUR FOOT PRINTS

The Connecticut Valley is also famous for fish fossils found in black shales. Plant impressions and insect tracks and burrows are also found.

The lava flow, now seen as prominent ridgelines overlooking the valley lowlands, formed as basalt oozed out of faults associated with the Eastern Border Fault. There are two major flows totaling 150 ft. thick in Greenfield and Deerfield, but they look like one layer since they occurred one after the other. Just to the south is the Holyoke Range, a 600 foot series of flows that form a prominent ridge that can be traced over 50 miles. The basalt has some radioactive minerals that can be dated due to their decay rate, we know these lavas solidified about 194 million years ago in the early Jurassic Period of the Mesozoic. This date is very useful in studying Connecticut Valley rocks since all the sedimentary layers below (such as found in Greenfield and Northampton) are older than this, and all the layers found above the lava (such as in Montague and Amherst) are younger. Thus the Connecticut Valley sequence is determined to be early Mesozoic -- from late Triassic through early Jurassic Periods.

Ancient earthquakes on the Eastern Border Fault tilted all the sedimentary layers as well as the basalt so they dip down toward the east at about a 25 degree angle. Keep this in mind as were look at events of the Cenozoic, our last (and continuing) Era of geologic time. Finally, by the end of the Mesozoic Era, 65 million years ago, the region had eroded to a flat plain called a peneplain. The only feature of great topographic interest at this time was Mt. Monadnock, which is notorious for withstanding the forces of erosion and remaining high (a "monadnock") while the surrounding region eroded to boring flatness.

CENOZOIC ERA

Two prominent events in the Cenozoic prominently shaped our land: uplift and glaciation. The former peneplain, a flat landscape almost at sea level, was uplifted about 1,000 feet in this area, resulting in extensive erosion. The Connecticut River developed its valley and the tilted lava flow, being more resistant than the surrounding sedimentary layers, became a prominent ridge.

Continental glaciers eroded the valley, too. Some soft sedimentary layers were scraped over 200 feet below the present land surface (subsequent deposits have filled in these low spots). Glacial scratching and polishing can be observed in many spots, especially in areas where the bedrock surface has been recently exposed.

The last glacier was at its maximum extent only 20,000 years ago, depositing glacial till and outwash gravels to form the coastal land from Long Island, NY to Martha's Vineyard and Nantucket. Cape Cod was formed in a similar way as the ice continued its retreat. When the glacier melted back to the Hartford, CT area, deposits blocked the whole valley, forming a dam. As ice melting continued, the dam created the great glacial lake named for the famous 19th century geologist and Amherst College President, Edward Hitchcock.

A CONTEMPORARY GLACIER AND GLACIAL LAKE; THIS IS HOW LAKE HITCHCOCK MAY HAVE APPEARED 16,000 YEARS AGO.

As the ice melted, the waters of Lake Hitchcock expanded up the Connecticut Valley, reaching the Franklin - Hampshire County area about 16,000 years ago. Great amounts of clay settled to the lake bottom, forming sediments known as varves, while streams brought loads of gravel and sand into the lake shore deltas.

CONTEMPORARY GLACIER WITH A DELTA OF GRAVEL AND SAND EMPTYING INTO A GLACIAL LAKE.

Today, the deltas are mined for their gravel.

CANOE BROOK SITE ALONG THE CONNECTICUT RIVER IN VERMONT. THIS IS A THICK DEPOSIT OF LAKE HITCHCOCK VARVES

Lake Hitchcock clay deposits (varves) have been mined for brick making and landfill linings.

The shoreline and lake bottom areas are prominently marked in the Greenfield area by terraces. The shoreline, just like a bath-tub ring, is preserved in the flat tops of deltas found at an elevation of 250 - 300 feet (above sea level) while the flat lake bottom is found at elevations of about 200 - 250 feet (note: these numbers change as one goes either north or south, due to glacial "rebound" effects). By the time it drained, about 14,000 years ago, with all its sediment layer fill, Lake Hitchcock was only about 50 feet deep.

With the lake gone, the Connecticut River came back to its valley and started to erode the Hitchcock sediments.

It has left abandoned channels (oxbow lakes) in many areas as floodplains were created and where the land was harder to erode, scenic narrow valley segments developed sometimes with waterfalls and rapids, great places for rock hunting, boating, and fishing.

OXBOW LAKE, HATFIELD, MA. To see what's under the river, click here.

REFERENCES: The following may be obtained at local libraries and bookstores or from the publisher: Earth View, c/o Richard Little, 6 Grand View Ln, Easthampton, MA 01027. Write for price list. Web Site: http://www.earthview.pair.com

<> Dinosaurs, Dunes, and Drifting Continents:the Geohistory of the Connecticut Valley
Richard D. Little, 1986, 2nd edition, 107 p. ISBN 0-9616520-0-4
Here is an up-to-date look at how the Connecticut Valley and Western New England rocks and landscape were created. Clear writing, many illustrations, references, and a bit of humor have made this an important resource for both the classroom and the general public.

<> NEW VIDEO PROGRAM: THE FLOW OF TIME
500 Million Years of Geohistory in the Connecticut River Valley
Richard D. Little, 1994, 45 minutes ISBN 0-9616520-3-9
With animations, dinosaurs, rift valleys, drifting continents, armored mud balls, and ice ages "come alive" as your host, Professor Richard Little, takes you on a geological tour of Western New England's exciting geological heritage. Original music by Greg Seaman. Narration by Amy Valinski.

<> Exploring Franklin County (Massachusetts) -- A Geology Guide
Richard D. Little, 1989, 100 p. ISBN 0-9616520-1-2
Direction to and information about over 30 sites such as waterfalls, caves, drumlins, old Lake Hitchcock, scenic views and includes a geology guide to Routes 2 and I-91. An excellent way to discover our scenic and exciting geohistory. (currently out of print)

<> The Rise and Fall of Lake Hitchcock - New England's Greatest Glacial Lake, Richard D. Little, 2000, 45 min. Find out about this amazing lake that has such an impact on the land and residents. Accompany a Tufts University class doing varve research, and see interviews with geologists discussing concretions, varves, groundwater, sand dunes, pingos, and other topics. See this website for more information: http://www.earthview.pair.com


Link to the Dinosaur State Park, Rocky Hill, Connecticut