Landforms

Tammy Marie Rittenour



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

Terraces

Approximately 12,000 years ago the sediment dam at Rocky Hill CT was eroded through, draining Glacial Lake Hitchcock. Initially a current was developed within the lake as the water level lowered. Upon reaching a depth in which most of the southern lake floor was exposed, the proto-Connecticut River began to flow. A number of small short-lived lakes remained in the deeper portions of the lake basin in Massachusetts and Vermont/New Hampshire. The proto-Connecticut River connected these lakes and eventually caused them to drain.

It is believed that Glacial Lake Hitchcock may have drained due to isostatic uplift, tilting the basin and causing a gradient for water to flow southward (Lewis and Stone, 1991). It was this gradient that initiated flow of the proto-Connecticut River. As isostatic uplift increased, the river eroded into the lake bottom to form a gentle gradient to the ocean, its base level. During incision, the river became perched on many bedrock and till nick points (or barriers to river erosion. These nick points prevented further erosion of the river and influenced terrace formation.

Since its inception, the Connecticut River has been a meandering river. In the past, as today, large sinuous meanders have flowed across the valley cutting a broad flood plain into the ancient lake floor.

When river erosion cuts through a nick point or there is additional isostatic uplift, the current flood plain is abandoned and the river incises farther into the lake floor to create a lower flood plain. The abandoned flood plains at higher elevations are called terraces.

Correlation of terraces throughout the valley has illuminated the evolution of the Connecticut River from its initial formation to its present course. Many nick points have been impacting the flow of the Connecticut River throughout its existence. One important nick point is the bedrock narrows between South Hadley and Holyoke Massachusetts.

THE HOLYOKE DAM IS BUILT ACROSS THE NICK POINT

Currently the gradient of the river changes from 0.03 m/km in the Hadley plain in the north, to 9.1 m/km within the narrow bedrock valley just north of Holyoke MA. This narrow valley was formed as the river slowly cut through lake sediments, till and bedrock. The river is still incising into the Mesozoic bedrock here as it tries to meet its most efficient gradient and slope profile. The modern floodplains and terraces north of the Holyoke Narrows are higher in elevation than those to the south due to this nick point. The Holyoke Dam was built to harness the river's energy in this location. The Turners Falls Dam was also built near a nick point in the Connecticut River valley. This nick point was one of the most important barriers to river erosion and has strongly affected the evolution of the Connecticut River in northern Massachusetts and Vermont/New Hampshire.

Waterfall at Turners Falls Massachusetts

During initial down cutting the Connecticut River became perched on a bedrock ridge near Turners Falls MA. Erosion continued south of this nick point but was suspended to the north. This situation created two magnificent waterfalls over the sandstone ridge. Prior to the construction of the Turners Falls dam and the flooding of Barton's Cove (then called Barton's Plain) in the late 1800's, deep ponds were located in the plunge pools of the former waterfalls. These ponds were called the Lily Ponds and the associated sandstone barrier that created the waterfalls was called the Lily Pond Barrier. Deep channels were cut into this bedrock barrier as the Connecticut River flowed over this nick point. Archeological evidence in northern Massachusetts from high terraces graded to the Lily Pond Barrier suggests that the waterfalls at Barton's Cove may have existed until about 9,000 years ago (personal communication, Dr. Dena Dincauze and Kathryn Curran, UMass Anthropology).

These waterfalls were later abandoned as the Connecticut River meandered south and found easily erodable sand and gravel at the end of the bedrock ledge. At this point the river quickly cut into these sediments to connect the elevation of the water north and south of the Lily Pond Barrier. This renewed erosion north of the Lilly Pond Barrier caused abandonment of the high terraces in northern Massachusetts and the formation of the current lower floodplain.

Sand Dunes

Immediately after Glacial Lake Hitchcock drained, the lake bottom sediments were exposed. Westerly winds picked up and blew the surface sediments into transverse and parabolic sand dunes .

These dunes are found primarily within the river valley on deltas, on the exposed lake bottom, and on the oldest river terraces. The age of these dunes has been determined by the OSL dating (Optically Stimulated Luminescence) of sand from the dunes. This dating procedure has produced an estimate for dune formation at approximately 12,200 11,800 years ago. Dated archeological sites found on various sand dunes have indicated that the dunes formed prior to 11,000 years ago (personal communication, Dr Dena Dincauze and Kathryn Curran, UMass Anthropology). These age estimates from sand dunes along with radiocarbon ages from varves indicate that Glacial Lake Hitchcock drained around 12,000 years ago.

Pingo Scars and Permafrost Features

Throughout the existence of Glacial Lake Hitchcock the climate in New England was cold and the vegetation was dominated by arctic plants. After lake drainage, the climate was either still extremely cold or the climate warmed and then returned to glacial conditions for a short period of time. In either case, the climate was extremely cold sometime after the lake drained.

These cold climate conditions froze the ground creating permafrost, much like that found in arctic regions today. Evidence for frozen ground conditions has been found in Connecticut as ice-wedge casts and polygonal patterned ground

(Stone and Ashley, 1992). These features formed as vertical veins of ice grew in frozen (permafrost) soils. Additional evidence for arctic conditions in New England has come from pingo scars located on the floor of Glacial Lake Hitchcock in Massachusetts and Connecticut. Pingos are large domes of soil and ice that form under permafrost conditions

. When the climate warmed, the ice domes melted, leaving a circular depression with slightly raised rims, or a pingo scar, on the landscape. Pingos require mean annual temperatures of less than 80C and are currently found in northern Canada and Alaska. The occurrence of pingos in the Connecticut River valley after 12,000 years ago provides important information about the post-glacial climate of New England.

References:

Lewis, R.S. and Stone, J.R., 1991, Late Quaternary Stratigraphy and depositional History of the Long Island Sound Basin:

Connecticut and New York: Journal of Coastal Research, Special Issue, v. 11, p. 1-23.

Stone, J.R. and Ashley, G.M., 1992, Ice-wedge casts, pingo scars, and the drainage of Glacial Lake Hitchcock: IN Robinson,

P. and Brady, J.B., eds., Guidebook for Field Trips in the Connecticut Valley Region of Massachusetts and
Adjacent States, v. 2 (84th Annual meeting of the New England Intercollegiate Geological Conference), Amherst
Massachusetts, University of Massachusetts, Geology Department, Contribution 66, p. 305-331.