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Geologists Search Bottom
The low-lying fields of the Perry Hodgdon farm just north of Randolph on Route 12 are lovely to any eye, but for a geologist, they have an extra attraction. These fields, geologists say, are a rare glimpse of the actual lake bottom of an enormous lake that dominated the regional landscape 10,000 years ago. We are talking a really enormous lake, 200 miles long, which may have lasted for 3000 years. It’s now called Glacial Lake Hitchcock after an Amherst College professor who had studied it. It was formed when retreating Ice Age glaciers left behind a sediment dam of rocks, soil, and debris as it retreated from the area now occupied by New Britain Conn. Earlier, the glacier had crawled in from the north, its enormous weight scraping away and pushing before it a nearly unimaginable amount of loose material. Eventually, however, its southern journey ended in New Britain, and it reversed course, retreating back to Canada. When it did so, it left behind a dam hundreds of feet high and hundreds of miles wide. This dam blocked the southerly flow of the waters in the northern New England watershed—the flow that is now the Connecticut River. Behind the dam, Lake Hitchcock began to form, eventually stretching up what we now call the Connecticut River valley all the way to St. Johnsbury. The water backed up also along the tributary rivers, such as the White and its branches—and even along smaller tributary brooks. Ayer’s Brook, which winds lazily alongside Route 12 north of Randolph, is one such brook that was inundated by Lake Hitchcock, becoming an inlet of water in a lake far bigger than today’s Lake Champlain. The water level apparently reached the level of the homes perched considerably above Route 12. When the sun was shining, the lake would have been the stunning aqua-blue color that even today characterizes glacial lakes around the world. And beside Ayer’s Brook—especially at the Hodgdon fields, geologists say—you can look at the land and say, "This is the preserved floor of the lake." New Research Those were the exact words spoken this Monday by Tufts University Prof. John "Jack" Ridge. Ridge is the latest of several geologists who have probed the soils there, digging with delight into clay they knew was deposited more than 10,000 years ago. Ridge’s project will be the most sophisticated geological testing yet. A substantial National Science Foundation grant will help him study special telltale soils like the clay underlying Perry Hodgdon’s cornfield. Ridge showed up early Monday morning with an undergraduate assistant, Robbie Bayless, and a drilling rig, and its owner and his assistant. They drove the rig down the little road that leads to a farm bridge over the brook, and perched on the streambank, about 25 feet above the water. During the thousands of years that the Hodgdon fields were at the bottom of Lake Hitchcock, brooks and winds brought in fine particles of clay, deposits that eventually built up to 100 feet or more in depth. Jack Ridge knows how to read those deposits. With the help of the NSF grant, he hopes to trace a story in the clays left behind throughout Lake Hitchcock that will help us understand one of the pressing issues of the day—climate change. We know that glaciers affect climate change—and vice versa—he explains in his grant application. But we can track the changes in climate and glacier movement only generally, in hundred-year segments. Ridge, incredibly, has learned to track glacial behavior year by year. The rig he brought to Hodgdon’s field propels a "hollow stem auger" which can extract a core of soil down to a depth of 130 feet. On a table by the riverbank, he displayed a small tubular section of material, pointing out a series of dark lines and lighter stripes. These, he explained, are almost the exact equivalent of rings in a tree, an annual record of weather and climate. The darker lines are winter deposits (mostly clay) and the lighter places are summer deposits (mostly sediments including organic matter). "Exposures of varves can be seen throughout the valley in river and stream cuts," says a University of Massachusetts website. "In the past, the varves were mined from clay pits to create the traditional red bricks seen in many of the historic buildings in the valley." Just as in trees, the spaces between the rings can tell a lot about the weather and climate at the time. A warm year will bring more meltwater into the lake, and more sediment, creating wider annual stripes, which scientists call "varves." A tree’s rings tell a story at best a century or two long. The varve patterns in Jack Ridge’s cores, though, tell a tale from 13,000 years ago. By the end of a long day Monday, Ridge and his crew had reached impenetrable material at 95 feet. They were up again at 7 a.m. the next day to drill an identical hole a few feet away. This is the second summer of the NSF grant, and it will take Ridge and crew to four or other sites in New England, tracing the edges of Lake Hitchcock. The Randolph site is the only one in Vermont. The chief expert on the configuration of Lake Hitchcock in central Vermont is Fred Larsen, professor of geology at Norwich University. Larsen, Ridge said, is "THE expert on glacial geology in central Vermont," and as a result he has spent a lot of time in Perry Hodgdon’s cornfield. So it was no surprise that Larsen turned up at the drilling site soon after work began Monday. Tuesday, too. On the Web To learn more, just Google "Glacial Lake Hitchcock." An unusually clear scientific description may be found at conn.river/hitchcock.html. |
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