Glacial Geology and Paleoclimate

Central Vermont was covered by 2+ miles of glacial ice just 20,000 years ago, and  students in this course work to unravel the record of ice advance and retreat, and to understand changes in regional and global climate. In the Dog River valley, home of Norwich University, and in nearby valleys, glacial meltwater was temporarily impounded and large lakes filled the valleys.  The water depth at the NU campus was on the order of 150-200 ft!

Below is a brief overview of research I have conducted, with colleagues, on the glacial geology of the Mad River valley, west of the Dog River. Students spent a great deal of time helping me tease out details of glacial retreat during numerous labs conducted in the Mad valley.

The image below is from a small sand and gravel pit in which the inside of a glacial-era lake delta is preserved. This particular delta built into a relatively small and high lake that we call Glacial Lake Granville that drained to the south, through the 1430′ (above sea level) Granville Notch.

This small Gilbert delta, found in the Hartshorn pit in a tributary to the Mad River, displays a topset-foreset contact that reveals the elevation of Glacial Lake Granville, which drained south through the Granville notch at 1410 ft above present sea level. Scale is in feet.

 

 

 

 

 

Simplified map of lake levels in the Mad River valley. When ice still filled much of the valley, small impounded lakes were formed in upper Clay Brook valley. With ice retreat exposing the southern part of the watershed a relatively small and high lake, Glacial Lake Granville, drained south through Granville Notch, at 1410 ft above sea level. As the ice margin retreated north a lower outlet opened to the north and a lower lake was established, Glacial Lake Winooski. (map data from Dunn and Springston, map created by Springston, G.)

 

 

 

 

The map to the right (click to enlarge) reveals the extent of ice-contact lakes that formed in the Mad River valley during glacial retreat. The glacial geology of the Mad River valley was mapped and reported on in:

Dunn, R.K., Springston, G.E. and Wright, S., 2011, Quaternary geology of the central Winooski River Watershed with focus on glacial lake history of tributary valleys (Thatcher Brook and Mad River): in, West, D., ed., New England Intercollegiate Geological Conference Guidebook, chapter C3, p. 1-32.

and for maps see (you can download free copies at the Vermont Geological Survey):

Dunn, R. K., Springston, G. E. and Donahue, N., 2007a, Surficial geologic map of the Mad River watershed, Vermont (northern sheet): Vermont Geological Survey Open File Report VG07-1A, 1 plate.

Dunn, R. K., Springston, G. E. and Donahue, N., 2007b, Surficial geologic map of the Mad River watershed, Vermont (southern sheet): Vermont Geological Survey Open File Report VG07-1B, 1 plate.

Some unusual conditions apparently existed in the Mad River valley during deglaciation. For example, very little deposition took place during Granville and Winooski times, especially of materials coarser than silt-clay. Varves, annual layers deposited in glacial lakes that undergo seasonal freezing-over, are characterized by clay with only silt partings at the most. Apparently, meltwater carried only limited, and fine, sediment into the valley. The image below shows some isolated coarse material deposited below glacial ice during retreat.

Glacial outwash in tributary to the Mad River. The gravel-sand outwash here is preserved in a tributary to the Mad R. and appears to have an unusual origin. Other deposits suggest ice was still present in this valley during deposition of the outwash and therefore it may have been deposited below the ice. I have proposed, tentatively!, that hydraulic jacking of the ice mass, by drainage of small lakes impounded up the valley, may have produced a meltwater path below the ice and a depocenter for this outwash (see Dunn, R.K., Springston, G.E. and Wright, S., 2011, Quaternary geology of the central Winooski River Watershed with focus on glacial lake history of tributary valleys (Thatcher Brook and Mad River): in, West, D., ed., New England Intercollegiate Geological Conference Guidebook, chapter C3, p. 1-32.)

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