Glacial Lake Hitchcock in the Valleys of the White and Ottauqueche Rivers, East-Central Vermont
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University of New Hampshire University of New Hampshire Scholars' Repository New England Intercollegiate Geological NEIGC Trips Excursion Collection 1-1-1987 Glacial Lake Hitchcock in the Valleys of the White and Ottauqueche Rivers, East-Central Vermont Larsen, Frederick D. Follow this and additional works at: https://scholars.unh.edu/neigc_trips Recommended Citation Larsen, Frederick D., "Glacial Lake Hitchcock in the Valleys of the White and Ottauqueche Rivers, East- Central Vermont" (1987). NEIGC Trips. 410. https://scholars.unh.edu/neigc_trips/410 This Text is brought to you for free and open access by the New England Intercollegiate Geological Excursion Collection at University of New Hampshire Scholars' Repository. It has been accepted for inclusion in NEIGC Trips by an authorized administrator of University of New Hampshire Scholars' Repository. For more information, please contact [email protected]. A - 3 GLACIAL LAKE HITCHCOCK IN THE VALLEYS OF THE WHITE AND OTTAUQUECHEE RIVERS, EAST-CENTRAL VERMONT by Frederick D. Larsen Department of Earth Science Norwich University Northfield, Vermont 05663 INTRODUCTION There are two main purposes of this field trip. One is to review Late Wisconsinan glacial stratigraphy at two exposures in West Lebanon, New Hampshire, where outwash older than the last glacial advance can be observed. The other is to address the question of the relative ages of glacial Lake Hitchcock, glacial Lake Winooski, and the Champlain Sea. Field trip stops will be made on the following U.S.G.S. 7.5-minute quadrangles: Hanover, Vt-NH; Quechee, Vt; Sharon, Vt; Randolph, Vt; and Brookfield, Vt. In addition, the field trip passes through the South Royal- ton, Vt, and Randolph Center, Vt, quadrangles. Erosion of com plex metamorphic rocks by streams and continental ice sheets has produced a rugged, hilly topography with 800 to 1,000 feet of local relief. Drainage is controlled by the Connecticut River which flows south-southwest through the area. Three major trib utaries of the Connecticut River in this area are the White, Ottauquechee and Mascoma Rivers. The area probably has been covered by ice sheets several times but specific evidence of multiple glaciation in east-cen tral Vermont and west-central New Hampshire has not been demon strated. Multiple-till exposures representing two separate gla ciations are known in northern, east-central, and southern New Hampshire (Koteff and Pessl, 1985), as well as in southern Que bec and southern New England. The margin of the last ice sheet retreated northward from Long Island at least by 19,000 years ago (Sirkin, 1982), and the Quebec Appalachians were deglaciated by 12,500 years ago (McDonald and Shilts, 1971). Therefore, the ice margin retreated through the field trip area between 19,000 and 12,500 years ago. Using linear interpolation and assuming steady rate of ice margin retreat, we can guess that the ice margin retreated through the area between 14,000 and 15,000 years ago. However, the ice probably had an increasing rate of retreat through this area. Stewart (1961), and Stewart and MacClintock (1964, 1969, and 1970) recognized three separate drift (till) sheets in Ver mont and westernmost New Hampshire. They named (1) a north- west-derived Bennington drift, (2) a northeast-derived Shelburne drift, and (3) a northwest-derived Burlington drift. Most of 30 Figure 2. A portion of James W. Goldthwait's glacial striations in New England; P, Putney, Vt., (Fl p . 60)• 31 A - 3 the area of this field trip lies in the area of the so-called Shelburne drift (Fig. 1). The Shelburne drift was found not to exist at its type locality (Wagner, Morse and Howe, 1972) and 8 of 9 indicator fans mapped in the area of the Shelburne drift are oriented to the south-southeast. According to Stewart and MacClintock, they should be oriented to the southwest of their source areas. I believe that the model of three drift sheets in Vermont and New Hampshire is untenable and that the surface till of New England resulted from,the advance and retreat of one ice sheet, the Late Wisconsinan (Laurentide) ice sheet. During retreat of the ice sheet in this area, the ice mar gin was accompanied by a northward-expanding glacial Lake Hitch cock (Lougee, 1939, 1957). Lake Hitchcock developed a stable outlet over a bedrock threshold at New Britain, Connecticut, and drainage down the present-day course of the Connecticut River was blocked by a large ice-contact delta at Rocky Hill, Connec ticut (Stone and others, 1982). Recent work by Koteff and Larsen (1985 and in prep) indicates that the former shoreline of Lake Hitchcock now rises toward about N21.5°W with a gradient of 0.90 m/km (4.74 ft/mi). The lake formed during ice retreat when the land was still depressed by the weight of the ice, and it ex tended 320 km from its spillway to West Burke, Vermont, before uplift due to the removal of the weight of the ice sheet com menced. Deltaic and lake-bottom deposits associated with Lake Hitchcock will be observed at several stops on this field trip. At its maximum extent, an arm of Lake Hitchcock extended up the valley of the Second Branch of the White River and into Williamstown Gulf in central Vermont. Gravel bars located in the valley bottom near East Brookfield appear to have been formed by the outlet stream from glacial Lake Winooski after Lake Hitch cock drained (Larsen, 1984). This would seem to indicate that the dam for Lake Hitchcock had been breached while glacial ice still blocked the northwest-draining Winooski River. However the level of Lake Hitchcock lowered, whether it was by a rapid breaching of its dam or by slow rebound of the crust, or by both, the Connecticut River eventually cut down through the sed iments of Lake Hitchcock leaving former flood plains elevated as stream terraces above the level of the modern flood plain. Stream-terrace deposits will be observed at Stops 1 and 2. No mention has been made of glacial Lake Upham in the above account. The reason for that is twofold. First, that portion of the map of Lake Upham shown by Lougee ( 1957) to be north of the so-called "Algonkian hinge line" essentially is the same as the shoreline shown by Larsen (1984) and Koteff and Larsen (1985 and in prep) to be the northern part of Lake Hitchcock. Second ly, although many data points (delta elevations) collected by Koteff and Larsen (in prep) fall below the level of Lake Hitch cock, at this time there is no clearly defined single shoreline that one might call "Lake Upham". For these two reasons, I sug gest that the term "Lake Upham" be dropped. 32 50 KM. Figure 3. Indicator fans in Vermont, New Hampshire, and south ern Quebec: 1. Barre; 2. B raintree; 3. Brocklebank; 4. Knox Mt. ; 5. Lebanon; 6. Glover; 7. Ascutney; 8. Cuttingsville; 9. Mt. Hereford; solid black, igneous source rocks; black lines down- glacier from plutons are 10% isopleths showing percent of source rocks in till samples. (Sources: 1, Larsen, 1972; 2-8, Great Pebble Campaigns at Norwich University, 1974-78 and 1980-81; 9, McDonald, 1967) 33 A-3 ADVANCE OF THE ICE % During advance of the last ice sheet in this part of the Connecticut Valley, braided meltwater streams issuing from the ice dropped tons of sediment in front of the ice as outwash or valley train deposits. As time went on, the ice sheet overrode these advance outwash deposits and in most areas removed them and recycled them into till or into younger advance outwash fur ther downstream. However, there are three places in east- central Vermont and western New Hampshire where advance outwash deposits can be observed. They are (1) along the Jail Branch southeast of Barre (Larsen, 1972, Stop 1), (2) on the south side of the White River east of Bethel, and (3) in two pits visited on this field trip in West Lebanon (Stops 1 and 2). Advance outwash probably occurs at other sites but exposures are not c ommon. DIRECTION OF ICE MOVEMENT The direction of movement of the former ice sheet can be ascertained by a study of striations, roche moutonnee forms, crag-and-tail features, indicator fans, and the orientation of elongated stones in till (till-fabric analysis). Figure 2 shows a portion of a map of glacial striations compiled by James W. Goldthwait (Flint, 1957). Note the fact that along the Connec ticut River of east-central Vermont and western New Hampshire striations trend between south-southwest and southeast. Away from the river both on the east and the west the striations are oriented between south-southeast and southeast. I attribute the pattern to one glaciation and to the facts that striations are made at different times during a single glaciation and that the direction of movement of bottom ice may be quite different than the regional gradient on the surface of the ice due to rugged topography. Support for the idea that ice movement in east-central Ver mont was to the south-southeast as shown in Figure 2 came from a series of "Great Pebble Campaigns" at Norwich University. Stu dents in physical geology labs each collected 100 pebbles at se lected sites around and south of granitic plutons and other unique source areas. When the percent of indicator clasts at each site was plotted it was found that the highest concentra tions were between due south and southeast of each source area studied (Fig. 3). When we compare the south-southeast orientation of the in dicator fan derived from the Lebanon done (5, Fig.