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
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
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.