Glacial Lake Hitchcock

Tammy Marie Rittenour



For inquiries contact Professor Julie Brigham-Grette, Department of Geosciences, University of Massachusetts Amherst

Formation of Glacial Lake Hitchcock

Approximately 18,000 years ago the southern margin of the Laurentide ice sheet began to retreat northward. During retreat a pro-glacial lake formed in the Connecticut River valley near Middletown CT (Glacial Lake Middletown). The abutting ice margin at Rocky Hill CT dumped large volumes of sand and gravel into this lake basin. When the ice margin retreated north of this position, the mass of sediments at Rocky Hill acted as a dam in the Connecticut River Valley. Water became impounded between this sediment dam (the Rocky Hill dam) and the retreating ice margin, creating Glacial Lake Hitchcock just prior to 15,000 years ago. The lake slowly grew as the ice continued to melt back and retreat up the Connecticut River valley. At its maximum extent, Glacial Lake Hitchcock expanded from Rocky Hill CT to St. Johnsbury VT (320 km or 200 miles).

In the early stages of Glacial Lake Hitchcock its water level was not stable. Initially Lake Middletown and Lake Hitchcock were connected and their water level was controlled by the Lake Middletown spillway. A subsequent drop in lake level separated the two basins. At this point a spillway for Glacial Lake Hitchcock formed in a low divide near New Britian CT (the New Britian Spillway). Water level slowly lowered from 35 m (115 ft) to 25 m (82 ft) above sea level as glacial till in the spillway channel was eroded. This stage of continued lake lowering is called the Connecticut Phase. The Stable Phase of Glacial Lake Hitchcock began after the New Britian Spillway became incised into bedrock, preventing further erosion and lake level change. The lake remained at this level for 2,000 3,000 years as the ice retreated from Connecticut into northern Vermont (Koteff et al., 1989).

Deltas

Deltas formed where fast-flowing streams entered Glacial Lake Hitchcock. Coarse sand and gravel were quickly deposited as streams entered the quiet lake environment, creating large deltas at the mouths of many of the tributaries entering the valley. Many airports have been built on these large flat surfaces (such as Westover Air Force Base and Bradley International Airport) and their sand and gravel is mined for road construction and concrete. Deltas have unique sedimentary structures that allow geologists to interpret past water levels of a lake. Fast-flowing streams and rivers have enough energy to transport coarse-grained sediments such as sand and gravel. When a stream enters a lake its water velocity is quickly decreased causing it to no longer have the energy needed to transport coarse-grained sediments. At this point these coarse sediments are dropped from the water column and deposited along an inclined slope (at the angle of repose) at the mouth of the river. This inclined slope is called the delta front and the dipping layers of sediment are called foreset beds. Fine-grained sediment is transported further into the lake and deposited in horizontal layers called bottomset beds.

With continual deposition at the mouth of a stream, a platform of sediments (delta) is extended into the lake. The stream is forced to flow over this platform to continue to reach the lake and delta front. This is called delta progradation the Mississippi River delta is an example of a prograding delta. As the stream flows over the foreset beds, horizontal topset beds are deposited. The contact between the topset and foreset beds is approximately at the lake water level.

Isostatic Uplift

Topset/foreset contact elevations have been measured in deltas throughout former Glacial Lake Hitchcock. These measurements have been used to determine past lake levels in the valley. When plotted on a longitudinal profile from Rocky Hill CT to St. Johnsbury VT, the past water level of Glacial Lake Hitchcock forms a plane that rises 0.889 m/km to the N200W (Koteff, et al., 1993). The tilt of this once horizontal water plane is due to isostatic rebound (uplift) of the land after ice retreat.

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 weight (ice) is called isostatic adjustment. In most locations throughout the former extent of the Laurentide ice sheet, isostatic rebound began immediately after ice retreat. However in New England, isostatic rebound appears to have been delayed by over 2000 years (Koteff et al., 1993). This delay in rebound can be seen in the longitudinal profile. The line marking the past lake level is straight indicating that the ice margin had retreated to northern Vermont, at the northern end of Glacial Lake Hitchcock, prior to isostatic rebound. If rebound had occurred during ice retreat (as seen at most areas, like the Great Lakes) then the lake-level profile would be curved and the rebound may have caused Glacial Lake Hitchcock to drain prior to achieving its maximum extent. A delay in isostatic rebound for thousands of years is unusual and at this point appears to be unique to New England.

Varves

During its existence, 15-12,000 years ago, Glacial Lake Hitchcock probably looked like many of the brilliantly blue glacial lakes seen today in Alaska and the Canadian Rocky Mountains .

The unusual color of these lakes is due to the suspension of fine silt and clay particles in the water. Fine particles absorb all but blue wavelengths, giving the water its blue appearance. This fine sediment associated with these lakes is called glacial flour. The erosive grinding action of a glacier reduces rocks to a fine powder. This glacial flour was transported in meltwater streams to Glacial Lake Hitchcock. When the stream entered the lake, coarse-grained sediments were deposited in the delta, while the fine-grained glacial flour was carried into the lake basin and deposited as lake sediment (or bottomset beds).

Within Glacial Lake Hitchcock, and many other glacial lakes, conditions were right to allow annual layering of the lake sediments. During the summer, abundant meltwater and sediments were transported into the lake and deposited as a layer of silt and sand on the lake bottom. In the winter the surface of Glacial Lake Hitchcock froze, allowing for fine clay-sized particles to settle out of the calm water. This clay formed a continuous layer that draped over the silt and sand deposited during the summer. Each couplet of a summer (silt and sand) and winter (clay) layer constitutes one varve and represents one year of deposition. Exposures of varves can be seen throughout the valley in river and stream cuts. 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.

Because each varve represents one year of deposition, the varves can be counted like tree rings to determine how long Glacial Lake Hitchcock existed. In 1922 Ernst Antevs, a Swedish geologist, traveled to New England to count, measure and correlate varves from Glacial Lake Hitchcock and the surrounding glacial lakes in the Ashuelot, Merrimack and Hudson River valleys. He counted 4,100 individual varves within the Connecticut River valley indicating that Glacial Lake Hitchcock had existed for at least 4,100 years (Ridge and Larsen, 1990).

Varve thickness varies with distance from the ice margin and from deltas, the two dominant sources of sediment input. Varves deposited near the ice margin are very thick (10 cm 1 m) because of the large volume of sediment released by the melting ice. Varves are also thick near deltas where a majority of sediment entered the lake. With increasing distance from the ice margin or deltas, varves become thinner due to the reduced supply of sediment to an area away from the ice margin.

Stratigraphically the first varves deposited in an area are the thickest because the ice margin is close. With time, the varves become thinner as the supply of sediment is reduced due to the thinning and retreat of the ice margin further north. This stratigraphy was seen in a core taken from the University of Massachusetts Amherst campus. Thick varves were recovered from 110 feet (33.5 m) below the land surface. These varves were deposited immediately after deglaciation in Amherst (~ 13,200 years ago). The thin varves were sampled from 10 feet (3.3 m) below the land surface. These varves were deposited 1000 years after the ice had retreated from the Amherst area (~12,400 years ago), while the ice margin was in central to northern Vermont/New Hampshire. Varves of medium thickness were collected from 25 feet (7.6 m) below the land surface and represent the average thickness of Glacial Lake Hitchcock varves in the Amherst area.

In addition to changes in varve thickness due to distance from the sediment supply, varve thickness also changes from year to year. These annual changes in thickness are due to: 1) local sediment input variations, and 2) regional sediment fluctuations that affect the whole lake. Examples of local sediment fluctuations are uneven sediment distribution at a delta front, increased sediment input from one tributary basin (drainage of a smaller lake up stream), and increased lake bottom erosion or deposition in one location.

Regional fluctuations in varve thickness are due to external processes such as climate and weather. Increased precipitation or temperature caused more runoff and meltwater to enter Glacial Lake Hitchcock. With heightened discharge, a larger volume of sediment was transported into the lake and a thicker varve was deposited that year. Current research at the University of Massachusetts Department of Geosciences is aimed at determining if cyclic pulses of sediment seen in Glacial Lake Hitchcock varves were caused by climate events in New England during late deglaciation (15,000 12,000 years ago).

Concretions in the Varves (Claystones)

Concretions, sometimes called claystones or mud-babies, are found in many varve sections throughout the Connecticut River valley. These concretions formed within the coarse summer layers sometime after varve deposition as calcium carbonate precipitated and crystallized around a nucleation point. Preferential growth of concretions in summer layers may have been due to the increased porosity and water content in the coarser sediment that allowed easy transfer of ions to nucleation sites. It is believed that the nucleation point of many concretions may have been decaying organic matter (Emerson, 1898).

The morphology (shape) of the concretions varies from place to place. The specific conditions that influence concretion shape at each varve locality are largely unknown. Possible factors may be the number of nucleation sites (i.e., variations in abundance of fossil organic matter), porosity of the sediments, and varve thickness. Research into the morphology and timing of concretion growth has been conducted by Al Werner and student Laura Levy at Mt. Holyoke College, South Hadley, MA.

References:

Emerson, B.K., 1898, Geology of old Hampshire county, Massachusetts, comprising Franklin, Hampshire and Hampdon

counties: US Geological Survey Monograph 29, 790 p.

Koteff, C., and Larsen, F.D., 1989, Postglacial uplift in western New England: Geologic evidence for delayed rebound:

In S. Gregresen and P.W. Bashman eds., Earthquakes at North-Atlantic Passive Margins:
Neotectonics and Postglacial Rebound, Kluwer Academic Publishers, Norwell MA, p. 105-123.

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.

Ridge, J.C. and Larsen, F.D., 1990, Re-evaluation of Antevs' New England varve chronology and new radiocarbon dates of

sediments from Glacial Lake Hitchcock: Geological Society of America, v. 102, p. 889-899.