About 16,000 years ago a huge lake formed in the Connecticut River Valley. The lake was born of the abundant glacial meltwater from the retreating ice sheet and held back from the sea by a natural dam at today's Rocky Hill, Connecticut. Edward Hitchcock, a professor of geology at Amherst College, is credited with first discovering the geological evidence of this ancient lake.
Lake Hitchcock was nearly 150 miles in length, stretching from Rocky Hill in the south to Lyme, New Hampshire in the north. During Lake Hitchcock’s 3-4000 year existence, the raging rivers and swollen streams of the glacial era brought large amounts of silt that settled onto the lake bottom forming the flat clay valley floor we today know so well. Each yearly cycle formed distinctive layers, or “varves” of clay. Geologists have counted the number of clay layers to estimate the life of the lake. This clay was proved to useful resource for numerous brick making operations here in the Valley.
These glacial streams and rivers also carried denser materials that, upon reaching the lake, settled out more quickly than the finer particles of clay. Great deltas were formed along the Lake's shores giving us today one of the Valley's most important geologic resources: gravel. As for the size of these now high and dry deltas... they may pale in comparison to the deltas of the Nile or the mighty Mississippi. But, at least one of these deltas is large enough to have built upon it most of the cities of Springfield, Chicopee, as well the towns of Ludow, South Hadley, and Granby. The Chicopee River we see today has merely cut a new path through this soft delta material until it hit bedrock and could go no further.
There is a curious aspect to these glacial deltas. As they were laid down along the entire length of Lake Hitchcock they could rise no higher than the water level of the lake itself. Thus, since the water level should have been at the same elevation above sea level throughout the entire lake, the delta tops, too, should today be all at the same elevation above sea level. But, this is not the case. As one travels north in the Valley, one finds that the tops of these deltas increase in elevation with the most northerly of them being nearly 500' higher above current sea level than those in the south. What could account for such a weird phenomenon?
Actually, no violation of the laws of geology or physics is required in the explanation. We must only remember that as seemingly solid as the ground beneath us appears to be, the earth's crust is actually floating on the earth's semi-liquid molten mantle, as does a boat on water. The more people in that boat, the more water it displaces and the deeper it sinks. Like that boat, the earth's crust, weighted down by nearly two miles of ice over thousands of square miles, began to displace and sink into the earth's mantle. Once free of this load, the pressure of the mantle caused the crust to rebound. The 500-foot greater rebound in the northern section of Lake Hitchcock is believed to be proportional to the thickness and, thus, the increased weight of the ice sheet in northern New England compared to the south.
About 12-13,000 years ago the "dam" at Rocky Hill, Connecticut was breached and Lake Hitchcock began to drain. Soon, vegetation, which already grew in the higher elevations, spread to the Valley floor. Over the next 12 millennia the Connecticut River and its tributaries cut through the soft delta and lakebed deposits first forming the floodplains then, at times, giant meanders. The Oxbow in Northampton is a striking example of just such a giant meander. A look at a topographical map shows others such as an older one in Hatfield.
The original lakebed, made primarily of layered clay, was not at all fertile and cannot itself account for the abundance of extremely fertile soil we find in the Valley today.
But, over 12,000 years, the decayed vegetation and nutrients that were carried by the river's floodwaters repeatedly washed over the growing floodplains and were deposited there. This process has given us some of the most fertile soil in the state. This soil has to be considered as much a geological resource as the gravel in the old glacial deltas or the abundant clay used for brick making... but it was not the gravel or the clay that drew the Natives and later the Europeans to the Valley.
"US-5: A Highway to History" © Robb Strycharz, 1996-2006
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