University of University of New Hampshire Scholars' Repository

New England Intercollegiate Geological NEIGC Trips Excursion Collection

1-1-1987

Southwest-Trending Striations in the , Central

Ackerly, Spafford C.

Larsen, Frederick D.

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Recommended Citation Ackerly, Spafford C. and Larsen, Frederick D., "Southwest-Trending Striations in the Green Mountains, Central Vermont" (1987). NEIGC Trips. 423. https://scholars.unh.edu/neigc_trips/423

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SOUTHWEST-TRENDING STRIATIONS IN THE GREEN MOUNTAINS, CENTRAL VERMONT

Spafford C. Ackerly Department of Geological Sciences, Cornell University Ithaca, New York 14853

Frederick D. Larsen Department of Earth Sciences, Norwich University Northfield, Vermont 05663

"That the highest points of land should have been scored by abrasions passing over them seemed to the older geologists better explained by floating than by glacial ice; for no one had then made clear how ice could move up hill to altitudes of thousands of feet. The ice of living glaciers moves down slopes: how then could the ancient ice have passed over the tops of the mountains unless the land itself had been so low that icebergs could have floated over them? The geologists had the credit of believing many strange stories, but even they hesitated to accept the doctrine that land ice could have been pushed over New England from the St. Lawrence valley."

Charles H. Hitchcock, 1904, Report of the Vermont State Geologist, p. 67

INTRODUCTION

In 1861 Edward Hitchcock reported two directions of glacial striations in the Green Mountains, in the vicinity of . One set, oriented to the southeast, roughly coincided with striation directions in other parts of Vermont, showing that currents of ice (or icebergs, in Hitchcock’s view) flowed southeast out of the Champlain Valley and across the mountains. The second set, however, directed to the southwest, was locally restricted to the valley of the Middlebury River and showed that ice flowed into the Champlain Valley. Hitchcock interpreted the southwest striations as evidence for local glaciers in the Green Mountains, though James Goldthwait argued in 1916 that the striations might have been produced by an ice sheet.

This field trip returns to Middlebury Gap, 116 years later, to reconsider the evidence of multiple striation directions in the Green Mountains. Recent mapping (work still in progress) shows that patterns of glacial striations in the Green Mountains are complex, recording multiple directions of ice movement.

REGIONAL PATTERNS OF GLACIATION

During the Late Wisconsinan glaciation, the Champlain Valley in western Vermont was a major conduit for ice flowing southward from ice domes in the Hudson Bay region. Ice radiated to the southwest into New York and to the southeast into New England from a locus in, or to the north of, the Champlain Valley (Mayewski et al., 1981; Hughes et al., 1985; Lowell and Kite, 1986). In Vermont, the southeast flow direction is indicated by patterns of

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glacial striations and indicator fans (Figs. 1-2). Indicator fans mapped by students at Northfield University show trends ranging from due south to S40E (Fig. 3). Many erratics of the reddish-brown Monkton quartzite have been carried southeast across the Green Mountains into southwest New Hampshire (Fig. 4).

The general pattern of deglaciation in Vermont was one of regional stagnation in the central highlands, and of oscillating or episodic ice margin retreat in the Hudson- Champlain Valley. The primary evidence for stagnation in central Vermont is the virtual absence of moraines and other ice-marginal features in highland areas. Ice-marginal features in the Hudson and southern Champlain Valleys, on the other hand, indicate active ice margin and stagnation zone retreat for the Champlain Valley lobe (Connally, 1970; De Simone and La Fleur, 1986). The Champlain Valley deglaciated rapidly, and was ice-free by about 13,000 years B.P. (Connally and Sirkin, 1973).

HISTORY OF ICE-FLOW STUDIES IN VERMONT

Striation patterns have been an important topic of study in Vermont since the early state surveys of the 19th century. Charles Adams (1846) and Edward Hitchcock and others (1861) noted the prevailing southeastward trend of striations across the state, though the cause of the pattern was controversial. Icebergs and "glacio-aqueous" agencies, acting in concert with epeirogenic movements of the land (submergence and elevation), were favored theories of 19th century geologists (Upham, 1894). Continental glaciation was widely accepted in the northeastern by the late 19th century, and Charles Hitchcock (1897) synthesized much of the striation data known at that time in his early description of the "Eastern Lobe" of the ice sheet. The "Eastern Lobe", now the Hudson-Champlain lobe, was considered as an eastern counterpart to the glacial lobes of the mid-western United States.

Multiple striation directions in Vermont have generated considerable controversy. As noted above, Edward Hitchcock (1861), and his son Charles Hitchcock(1904), considered multiple striation directions near Middlebury Gap as evidence for local glaciation in the Green Mountains. However, James Goldthwait (1916, p.68) argued that multiple striations, "though so strange as to excite much speculation in the ’60's", could be produced along the lobate margin of an ice sheet. Stewart and MacClintock (1969) considered multiple striations as evidence for multiple ice advances across Vermont, both from the northeast and from the northwest, but this concept has not been substantiated by later workers (Larsen, 1972; Wagner et al., 1972).

RESULTS OF RECENT WORK

Recent mapping has revealed a rather large region of southwest-trending striations in the Green Mountains extending for at least 60 kilometers along the crest of the range from Appalachian Gap in the north (44°12'30") at least as far south as Sherburne Pass (43°40') (Figs. 5 and 6). One of the best and most accessible places to observe the southwest striations is at Middlebury Gap where they were first reported by Hitchcock and others in 1861 (Fig. 7). Early attempts to locate striations to the north and to the south of Middlebury Gap were thwarted by a thick cover of till on the Green Mountain summits (Upham, 1889; Hitchcock, 1904). Much of the present mapping was possible because of recent bedrock exposures on the Long .

The crest of the Green Mountains in central Vermont lies mainly above 2,000 feet, and

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Figure 5. Glacial features in northern Vermont. Southwest striations occur along the crest of the Green Mountains from Appalachian Gap (AG) south to Shelburne Pass (SP) (MG = Middlebury Gap) (sources: Stewart and MacClintock, 1970; Connally, 1970; Wagner, 1970).

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Figure 6. Topography and patterns of glacial striations in the Green Mountains, central Vermont. Striation location at dot or arrow; arrows show direction of crag and tail features. Numbers refer to STOPS in field trip guide. "P" is location of 2 ft. diameter pothole near summit of Burnt Rock Mtn. (Doll, 1936).

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is flanked on the west by the Champlain Valley lowlands, and on the east by the Valley and the Northfield Mountains. North-south trending valleys and ridges are the products of differential erosion of metamorphic rocks of the Green Mountain Anticlinorium. The range is breached by the west-flowing Winooski and Lamoille Rivers (superimposed drainages), and roughly north-south trending tributaries to these rivers form a crude trellis drainage pattern (Figs. 5 and 6).

The predominant striation direction in northern Vermont is to the southeast (Figs. 2, 5) except along the very crest of the Green Mountains between Appalachian Gap and Sherburne Pass where striations are to the southwest (Figs. 5, 6). Southeast striations on the highest summits (Mt. Mansfield, Camels Hump, the Northfield Mountains, Killington Peak) are generally oriented S40E to S50E. At lower elevations, striation directions show some conformity to valley bottoms (as in the Valley, Fig.6) but striations may also be oblique to valley trends (as in the Mad River Valley, Fig. 6). Peaks cause some deflection of ice flow, as at Camel's Hump where flow on the peak is to the southeast, shifting to east-west in the col about two kilometers to the south.

Southwest striations occur primarily along the very crest of the Green Mountains and locally in the Mad River and Champlain Valleys. The boundary between southeast and south west-trending striations near Appalachian Gap is abrupt, changing from about S60E north of the gap to about S70W south of the gap, in a distance of less than three kilometers. Following south along the range, the southwest striation direction changes gradually to about S50W near Middlebury Gap and about S30W near Sherburne Pass (Fig. 5). Cross-cutting relationships of striations in the vicinity of Middlebury Gap, and at one locality near , show that southwest striations on the crest of the Green Mountains are younger than (cut across) southeast striations. This is opposite to what Stewart and MacClintock (1969) and Connally (1970) report in the Champlain Valley, where southwest striations are apparently older than southeast striations. There is no evidence for southwest movement of clasts in the Braintree indicator fan, located 21 kilometers east-northeast of Middlebury Gap (Figs. 6 and 8).

DISCUSSION

■ The sequence of glacial, or deglacial, events that produced the striation patterns in the Green Mountains is seemingly complex. The regional pattern of striations in the Green Mountains, and the dispersion direction of erratics in central Vermont, indicate ice flow to the southeast from a source area in the St. Lawrence and Champlain Valleys. However, at least along the very crest of the Green Mountains, from Appalachian Gap south to Sherburne Pass, southwest striations show that the last flow of ice was into the Champlain Valley. At least locally, the ice surface also must have been sloping into the Champlain Valley. Either there was a local buildup of ice in the central Green Mountains, or a drawdown of ice in the Champlain Valley. There is no compelling evidence for a local ice cap in this region.

Studies of striation patterns in many areas of northern New England and adjacent Canada show that local ice flow reversals were common during retreat of the last ice sheet (eg. Lamarche, 1974; Chauvin et al., 1985; Lowell and Kite, 1986). The mechanism of flow reversal is mainly related to rapid thinning of ice in ice-streams and ice lobes, especially in the St. Lawrence Valley. Thinning and drawdown of ice in the Champlain Valley could account for local reversals of flow to the southwest along the Green Mountain crest, thereby explaining the localized distribution of the southwest striations immediately adjacent to the Champlain Valley. At least for a short time there was probably an ice divide in the region of

375 Figure 7. Striation patterns in the Middlebury Gap area. Striation location at dot or arrow; arrows show direction of crag and tail features. Numbers refer to STOPS in field trip guide. The common occurrence of southeast striations at Middlebury Gap is probably because the gap is in the lee of significant bedrock knobs to the northeast: Burnt Hill, Silent Cliff and the Hat Crown.

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Figure 8. Distribution of indicator clasts in northwest part of Randolph 15' quadrangle, (A) Braintree indicator fan based on percent of total igneous pebbles (granite, diorite, and gabbro); stipple pattern, Braintree Pluton, (B) indicator fan based on percent of diorite pebbles; stipple pattern , area of diorite outcrop, (C) distribution of chlorite-bearing pebbles derived from the Cambro-Ordovician,-€0; DSn, Northfield Formation; Dwr, Waits River Formation; R.M.C., Richardson Memorial Contact, (D) distribution of pebbles of brown-weathering calcareous quartzite derived from the Waits River Formation.

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the Mad River Valley, with ice flowing both to the southeast and to the southwest. The divide may have sloped to the south, resulting in the more southerly component of the southwest flow near Sherburne Pass, but more work must be done in this area. The hypothesis outlined above is anticipated by the theoretical ice surface reconstructions of Hughes et al. (1985, Fig. 3) in their model for about 16,000 years B.P. showing flowlines diverging to the southeast and to the southwest in central Vermont.

The hypothesis of ice drawdown in the Champlain Valley, combined with localized restructuring of the ice sheet, raises a number of important, and as yet unanswered, questions. Where was the southern margin of the ice sheet when ice flowed southwest into the Champlain Valley? Did ice in the Champlain Valley remain active after the mountains became ice-free? Did the margin of southwest flow retreat up the west flank of the Green Mountains, contributing sediment to kame deltas in the Champlain Valley (see Connally, 1982), or did the southwest flow stagnate in a "top to bottom" mode, essentially collapsing in place? Does the "boundary" between southeast and southwest flow directions near Appalachian Gap represent the southern margin of late-stage ice flowing southeast across the Green Mountains in northern Vermont? These questions provide the focus for on-going studies in the Green Mountains.

ACKNOWLEDGMENTS

We thank Stewart F. Clark for unpublished data on striation directions in the Northfield Mountains.

REFERENCES

Adams, C.H., 1846, Second annual report on the geology of the State of Vermont, Chauncey Goodrich, Burlington. 267 p.

Chauvin, L., G. Martineau, and P. LaSalle, 1985, Deglaciation of the lower St. Lawrence region, : Geological Society of America, Special Paper 197, p. 111-123.

Christman, R.A., and D.T. Secor Jr., 1961, Geology of the Camels Hump quadrangle, Vermont: Vermont Geological Survey, Bulletin 15, 70 p.

Connally, G.G., 1970, Surficial geology of the Brandon-Ticonderoga 15-minute quadrangles, Vermont: Studies in Vermont Geology, No. 2, 27p.

Connally, G.G., 1982, Deglacial history of western Vermont: in, Late Wisconsinan glaciation in New England, G.J. Larson and B.D. Stone, eds., Kendall/Hunt, Dubuque, Iowa, p. 183-193.

Connally, G.G., and L.A. Sirkin, 1973, Wisconsinan history of the Hudson-Champlain Lobe: Geological Society of America, Memoir 136, p. 47-69.

De Simone, D.J., and R.G. La Fleur, 1986, Glaciolacustrine phases in the northern Hudson lowland and correlatives in western Vermont: Northeastern Geology, v. 8, p. 218-229.

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Doll, C.G., 1936, A glacial pothole on the ridge near Fayston, Vermont: Report of the Vermont State Geologist, 1935-1936, p. 145-151.

Flint, R.F., 1957, Glacial and Pleistocene geology: Wiley and Sons, New York, 533 p.

Flint, R.F., 1971, Glacial and quaternary geology: Wiley and Sons, New York, 892 p.

Goldthwait, J.W., 1916, Evidence for and against the former existence of local glaciers in Vermont: Report of the Vermont State Geologist, 1915-1916, p. 42-73.

Hitchcock, C.H.,1897, The eastern lobe of the ice-sheet: American Geologist, v. 20, p. 27-83.

Hitchcock, C.H., 1904, Glaciation of the Green Mountain range: Report of the Vermont State Geologist, 1903-1904, p. 67-85.

Hitchcock, E., et al., 1861, Geology of Vermont, 2 vols., 988p.

Hughes, T., H.W. Boms, Jr., J.L. Fastook, J.S. Kite, M.R. Hyland, and T.V. Lowell, 1985, Models of glacial reconstruction and deglaciation applied to maritime Canada and New England: Geological Society of America, Special Paper 197, p. 139-150.

Lamarche R.Y., 1974, Southeastward, northward, and westward ice movement in the Asbestos Area of southern Quebec: Geological Society of America, Bulletin, v. 85, p. 465-470.

Larsen, F.D., 1972, Glacial history of central Vermont: Guidebook for Fieldtrips in Vermont, 64th Annual Meeting, New England Intercollegiate Geological Conference, p. 297-316.

Lowell T.V., and J.S. Kite, 1986, Glaciation style of northwestern : in Contributions to the quaternary geology of northern Maine and adjacent Quebec, J.S. Kite, T.V. Lowell, W.B. Thompson, eds., Maine Geological Survey, Bulletin 37, p. 53-68.

Mayewski, P.A., G.H. Denton, and T.J. Hughes, 1981, Late Wisconsin ice sheets in North America: in The last great ice sheets, G.H. Denton and T.J. Hughes, eds., Wiley and Sons, New York p. 67-170.

Stewart, D.P., and P. MacClintock, 1969, The surficial geology and Pleistocene history of Vermont: Vermont Geological Survey, Bulletin 31, 251 p.

Stewart, D.P., and P. MacClintock, 1970, Surficial geologic map of Vermont, C.G. Doll, ed., Vermont Geological Survey.

Upham, W., 1889, Ascents of Camel's Hump and Lincoln Mountain, Vermont: Appalachia, v. 5, p. 319-326.

Upham, W., 1894, The epeirogenic theory of the cause of the ice age: The Glacialists' Magazine, v. 1, p.211-217.

Wagner, P., J.D. Morse, and C.C. Howe, 1972, Till studies, Shelburne Vermont: Guidebook for Fieldtrips in Vermont, 64th Annual Meeting, New England Intercollegiate Geological Conference, p. 377-387.

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ITINERARY

Assembly point is at the U-Haul garage on the east side of Vermont Route 100 at the junction with the Lincoln Gap Road, located about 19 miles south of both Exits 9 and 10 on Interstate 89 and about 0.4 miles south-southwest of the Warren General Store. Assembly time is 9:00 A.M. Topographic maps: Warren, Hancock, and Bread Loaf 7.5 minute quadrangles.

Mileage

0.0 STOP 1: The outcrop at the junction of the Lincoln Gap Road and Route 100 shows striations oriented S43W on stoss surfaces, and meltwater scour on lee surfaces, probably representing pressure-melting and regelation (freezing) processes at the base of the ice. These SW striations may or may not be related to SW striations along the crest of the Green Mountains to the west. Bedrock is Cambrian Pinney Hollow formation (Doll, 1961).

STOP 2 (Optional): 5.3 miles north of STOP 1 on Route 100, at bridge across the Mad River, just south of the village of Irasburg. Strong striations to the SE, with large crag and tails.

STOP 3 (Optional): Follow Lincoln Gap road west from STOP 1 for 2.6 miles. Turn right just beyond mailbox and brook crossing, follow narrow dirt road to pit.

The Hartshorn pit is located in a small delta on the projected shoreline (N20W-0.9m/km) of glacial Lake Granville that formed to the north of a 1,410-foot threshold at Granville Notch. Deltaic topsets consist of 2.0 meters of poorly sorted pebble gravel with cobbles interbedded with layers of fine sand, medium sand, and pebbly coarse sand with dune crossbeds dipping south. Deltaic foreset and collapsed beds are greater than 2.0 meters thick under a trimmed surface and dip to the south. The foreset beds consist of layers of poorly sorted fine to medium grained sand and pebble gravel.

Proceed south on Route 100, from Stop 1.

0.1 Take a right (west) on a small dirt road immediately before (north of) a bridge over the Mad River. Keep to the right and enter into gravel pit.

0.3 STOP 4: The Don Moore pit is made up of two active faces. The higher northwest face is about 160 meters (525 ft) long and up to 27 meters (89 ft) high. The lower southwest face is about 81 meters (265 ft) long. A measured section located near the center of the northwest face has four main units in a fining-upward sequence that is capped by colluvium. Unit 1 at the base is poorly sorted pebble gravel with cobbles and is interpreted to have been formed in a subglacial tunnel that drained south into glacial Lake Granville.

Unit 2 is 8.7 meters thick and is composed mainly of interbedded fine and very fine sand. Ripple crossbedding dips both to the northeast and southwest. In the middle of Unit 2 are 1.6 meters of granule to pebble gravel interbedded with fine sand. Unit 2 is interpreted to be proximal lake-bottom sediments deposited near the ice margin.

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Unit 3 is 4.5 meters thick and is composed of fine sand in massive grain flows interbedded with very fine sand. A-type ripple cross-lamination near the middle of several grain flows indicates transport to the northeast. Unit 3 is interpreted to have been deposited by turbidity currents possibly kicked into action by meteoric events in tributary valleys to the south or west.

Unit 4 consists of 3.1 meters of laminated gray silt and orangish-brown to brown, fine to very fine sand in beds about 1 centimeter thick. Minor crossbeds in fine sand dip to the southwest. The inferred environment of deposition was the quiet bottom of glacial Lake Granville when the ice margin was located well to the north. The depth of Lake Granville was about 115 meters when the uppermost lacustrine beds were deposited.

The pebble gravel with cobbles that caps the western half of the southwest face underlies a stream terrace that extends 90 meters to the west. This terrace was formed after Lake Granville drained and may have been graded to Lake Winooski. Sediments under the stream-terrace gravels have crossbeds that dip toward S10W to S25W, up the Mad River valley. In the past, 2.0 to 3.0 meters of south-dipping trough crossbeds 20 to 50 centimeters thick were exposed in the southwest face.

Proceed back to Route 100.

0.5 Turn right (south) on Route 100, cross the Mad River.

6.5 Height of land at Granville Notch. Spillway of Lake Granville at an approximate elevation of 1,410 feet.

7.2 Potholes(?) in the left (east) wall of the notch.

10.6 Town of Granville. Continue south on Route 100. The valley floor south of Granville is the outwash(?) plain for glacial meltwaters flowing through Granville Notch.

14.9 Pass outcrop on right (west) side of road, immediately north of the junction of Routes 100 and 125.

15.0 Turn right (west) onto Route 125 and park on right (market on south side of road).

STOP 5: Outcrop on the west side of Route 100, immediately north of junction with Route 125. Striations are S50E.

% 18.0 Pass road to Texas Falls picnic area (nice potholes). Continue west on Route 125.

19.8 Park in pullout on left (south) side of road, immediately uphill (west) of glacially polished outcrop on right (north) side of road.

STOP 6: Ascend up and to the left (west) of the roadcut about 40 feet to a small slab showing striations (with stoss and lee) trending east-southeast (S85E) and southwest (S53W). The SW striations cut across the SE striations.

A section of the old Middlebury Gap Road (?) can be followed from the pullout uphill to the west for about a half mile. In 1987 a slump along the river exposed laminated silts and fine sands containing small dropstones.

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Continue west on Route 125.

21.2 Park at the height of land, Middlebury Gap (elevation 2144 feet).

STOP 7: Hike south on the , pass top of ski lift, pass shelter (0.3 miles), come to ski trail just beyond shelter.

STOP 7 A: Good SW striations at top of this ski trail about 150 meters east of Long Trail. Continue on Long Trail about 0.1 miles to junction, take side trail west for 0.1 miles to to Lake Pleiad.

STOP 7B: South shore of Lake Pleiad, SE striations and grooves (stoss and lee) crossed by weathered SW striations. Return to Long Trail and continue south. Cross ski trail and continue upwards. Cross another ski trail.

STOP 7C: Excellent SW striations on ledges, at ski trail crossing. Continue up to top of spur. Cross two ski .

STOP 7D:. Strong SW striations at second ski trail crossing; SW striations predominate at higher elevations to south along the range. Leave Long Trail and follow ski lift-line down and west to lookout near Worth Lodge.

STOP 7E: View N of Lake Pleiad and Middlebury Gap. Return to Middlebury Gap.

STOP 7F: Walk north on Long Trail to Burnt Hill, strong striations to the SW.

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