ICE-AGE LAKE UNDER CONSTRUCTION

Ed Klekowski and Andrew Wier Sport Diver, Volume 5, No. 4, pp. 14-15, August, 1997


Lake Hitchcock, fed by glacial melt water for approximately 3,000 years, disappeared from the New England landscape about 12,000 years ago. Glacial lakes such as Lake Hitchcock formed geological deposits known as varves, each varve is composed of a couplet consisting of a whitish-gray clay layer and a yellowish-brown silty/sandy layer; a couplet represents a single year, the silty/sand deposited in the short summer during ice melt and the clay layer deposited in the long winter when the lake was frozen.

Although Lake Hitchcock disappeared before the first Amerindians entered the Connecticut River Valley, divers can still explore its waters; and that is what we were doing, forty feet under the Connecticut River, exploring a lake that vanished. The river's current had torn a small underwater canyon through a large block of lake sediment. We swam gently down this cleft, the orderly varves were a time warp back to the ice ages. As we descended deeper and deeper, year after year, decade after decade, and, finally, forgotten centuries passed and were lost from view. What events did they chronicle? The birth of a mastodon, the roar of a saber tooth cat, or perhaps something more prosaic but certainly more important -- the survival of the first tree seedling after the ice retreat.

At 60 feet the varve layers abruptly came to an end. With wetsuits streaked with gray glacial clay, we knelt on the bottom and looked up. Surrounding us was an underwater amphitheater of varves with the upper layers disappearing into darkness. Stacks of clay occasionally crumbled and fell, leaving a plume of gray "smoke" as the clay particles dissolved into the water. Bass and other fish were attracted to these small underwater avalanches in hopes of catching the animals inhabiting the varves.

Lake Hitchcock was one of the largest of the glacial lakes in New England. It stretched from mid-Connecticut to northern Vermont, approximately 175 miles. The impoundment resulted from glacial deposits at Rocky Hill, Connecticut that dammed the ice melt as the last glacier retreated northward. The Rocky Hill dam was breached and the lake drained about 12,000 years ago. The Connecticut River generally follows the course of Lake Hitchcock and, in many places, the river has eroded into and sometimes through the lake-bottom sediments. Diving in these portions of the river is, in many respects, like going back in time and exploring the bottom of Lake Hitchcock.

Descending through the river's waters to the lake sediments, the first varves often appear as ghostly white sheets of clay. Closer examination of these clay surfaces reveals a Swiss-cheese-like texture caused by countless burrows of chironomid larvae, an as yet unknown species of the genus Axarus. Chironomids are midges (insects) whose larvae are aquatic, contain hemoglobin, and are an important component of freshwater food webs. Breaking up a piece of clay releases these red worm-like larvae.

In the deeper part of the river, erosion has cut into the varves and they can be viewed in cross-section. Often the sandy/silty layer of a varve couplet is eroded and undercuts the clay layer. Looking carefully with a dive light into these crevices may reveal a pair of antennae sensing the environment. Looking closer, the diver will find a pair of stalked compound eyes looking back! The Connecticut River crayfish (Orconectes limosus), a solitary bottom dweller, spends its day hiding in crevices or under stones in the river. Its food includes insect larvae (chironomids), smaller crustaceans and dead animal matter. Crayfish are the food of choice for the many carnivorous fish in the river, thus this crustacean's reclusive nature is not surprising.

Very curious concretions of organic matter and carbonate are embedded in the clay portions of the varves and literally carpet the river bottom downstream of exposed and eroding Lake Hitchcock sediments. These concretions are tabular, approximately 6-8 inches across, and resemble puzzle pieces with circular holes punched in them; no two are identical and almost all are very attractive with sensuous, sinusoidal curves. All Lake Hitchcock divers return with a handful as mementos of their visit.

Each varve couplet marks a year; a continuous series of varves is the geological equivalent to the annual growth rings of a very old tree. The thickness of the couplet layers is a record of past environments. The varve couplets beneath the Connecticut River have never been studied and thus may offer new information concerning the rate of ice retreat and climate changes as we exited the last ice age.

In the Northeast the glacier began to retreat about 21,200 years ago. In its wake a series of freshwater glacial lakes were formed. The largest of these was glacial Lake Connecticut, which later became Long Island Sound when sea level rose. Two glacial lakes are traversed by the Hudson River: Lake Hudson in the south and Lake Albany in the north. In New Hampshire the Merrimack River follows the bed of glacial Lake Merrimack. In all of these sites, divers should discover ice age lake sediments similar to those under the Connecticut River.