Lake Hitchcock: An Introduction
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Lake Hitchcock was a glacial lake that formed approximately 18,000 years ago in the Connecticut River Valley. After the Laurentide ice sheet retreated, glacial melt water was trapped behind a large sediment dam which blocked up the Connecticut River, creating the long, narrow lake. The lake existed for approximately 4,100 years. At its longest, Lake Hitchcock stretched from the moraine dam at present-day Rocky Hill, Connecticut, to West Burke, Vermont 250 miles long and up to 20 miles wide.
The Connecticut River Valley possesses an interesting and significant geological history. As geologist Richard Little points out, it includes "oceans, mountains, earthquakes, dinosaurs, lava flows, glaciers, rivers and people." It is,"one of the few places in the world where such diverse history can be seen in so small a geographic area." (Little, Dinosaurs, Dunes and Drifting Continents)
Glacial Lake Hitchcock, named after American Geologist Edward Hitchcock (24 May 1793 – 27 February 1864)
During the retreat of the last glacial sheet through New England (18- 14,000 years ago), melt-water streams from the retreating glacier dumped large volumes of sand and gravel in the New Britain - Rocky Hill CT area to create a dam. As the glacial ice melted (retreated), water became trapped behind this sediment dam forming Lake Hitchcock, which, over the course of about 4,000 years, gradually extended up the Connecticut River drainage as far north as West Burke, VT, a distance of about 250 miles and was as much as 20 miles in width. Eventually, the 3 mile deep dam broke, the lake waters drained southward, and the modern Connecticut River was established on the drained bed of glacial Lake Hitchcock.
BUT WAIT! How can a lake exist ON A HILL?: Governed by a height of 75ft above sea level by the spillway in New Britain CT, The lake, went from Rocky Hill CT (elevation 223 feet) past St. Johnsbury VT (elevation 614 feet). HOW can a lake have a tilted shoreline? Easy… The tilt of this once horizontal water plane is due to uplift of the land after the ice retreat.
Let’s remember some 5th grade geology. The earth is comprised of a molten center of liquid rock with a crust floating on that liquid. Now let’s look at some math. A cubic foot of water (or ice) weighs aprox. 62.42796 pounds. With an average depth of, let’s say ½ mile (Ice was thicker in the north, thinner in the south) the weight per square mile of land would have been at least 4,225,317,212,160 pounds (or 2.11 billion tons per square mile). Now that’s just ice, not the additional weight of glacial debris.
During glaciations, the weight of continental ice sheets depressed the land beneath them. With the retreat of the ice, newly exposed land rebounded from its depressed position. This shift in the land surface due to the addition and removal of the weight of the ice is called isostatic adjustment, or Glacial Rebound. Normally, the land would rebound immediately after ice retreated, however in New England, isostatic rebound appears to have been delayed by over 2000 years (Koteff et al., 1993).
BONUS: While this formation was in place well before the Wisconsin Ice Sheet moved across the region, it's worth a mention. For Northbound travelers, a bit tough. Bring Binoculars and look across the highway at the road cut. This is a unique piece of geology. While the rocks look like basalt and are really close to the basalt of the Holyoke Range, they are mostly due to ash falls and stream deposits.
For Southbound travelers, walk to the western edge of the parking area to get a hands on, up close look. Can you see the layers and conglomerate of stream debris? The north south face of the rock appears smooth, but where the rock face is east west the layers are more apparent. Can you guess which direction the flow went? These sedimentary layers are very basalt looking because all the grains are from local basalt and they did not get transported very far, so sorting and layers are not well displayed.
To qualify for this EarthCache go to the posted coordinates of either the Northbound or Southbound rest stops and complete the following tasks (You are approximately 200 feet above the river):
1) While the dam at Rocky Hill is approximately 50 miles south and at 60ft above sea level what is your current elevation? Calculate the slope of the current valley (rise/run).
2) St. Johnsbury Vt is 170 miles to the north and at 600 feet elevation. Calculate the slope from your current position to St. Johnsbury VT
3) Based on your slope comparisons, was the post glacial uplift equal in both north and south directions? (We will review this question in the Earthcache: Lake Hitchcock: Plume falls and Nick Points)
4) Estimate the height of the road cut basalt ash flow formation described in the bonus
REFERENCES:
Dinosaurs, Dunes, and Drifting Continents:the Geohistory of the Connecticut Valley Richard D. Little, 1986, 2nd edition, 107 p. ISBN 0-9616520-0-4
The Rise and Fall of Lake Hitchcock - New England's Greatest Glacial Lake, Richard D. Little, 2000 ISBN 961652055 9780961652050
Koteff, C., Robinson, G.R., Goldsmith, R., and Thompson, W.B., 1993, Delayed postglacial uplift and synglacial sea levels in coastal central New England: Quaternary Research, v. 40, p. 46-54.
Additional Resources
http://www.bio.umass.edu/biology/conn.river/crvgeology.html
http://en.wikipedia.org/wiki/Lake_Hitchcock
http://www.daukas.com/Geoscience/MAtour/CTV/background.html
http://www.bio.umass.edu/biology/conn.river/hitchcock.html
http://www.wesleyan.edu/ctgeology/Glacial/GlacialGeology.html
Additional Hints
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