Some late Wisconsin and Post-Wisconsin Shore-lines of North-western Vermont.

The investigation upon which this paper is based was made during the summer of 1906. The work was done as J. D. Whitney Scholar for 1906, under the direction of Professor Woodworth, to whom I am indebted for many suggestions, especially as to field methods and interpretations. The field expenses were paid by Harvard University from the Josiah Dwight Whitney fund for geological field study. Mr. Harold C. Durrell of Cambridge, Mass., accompanied me at his own expense during most of the summer, very graciously assisting me in many ways.

The district studied includes chiefly that portion of Vermont which lies north of the middle of the state, and west of the divide between

Lake Champlain and the Connecticut River. The work was extended to Shefford Mountain and Brome Mountain in southern Quebec.

The chief rocks of this district are metamorphic. The dominant structures, belts of gneiss, schist and limestone, strike N. N. E. After the surface of these rocks had been maturely dissected, it was glaciated and, along the western part, depressed below sea-level, and partly buried by marine and fresh-water deposits which are largely of glacial origin. In many of the valleys further east there are large bodies of water-laid glacial materials. Changes of local water-levels during the retreat of the ice-front, and successive elevations of the region (of which the area under consideration is a part) have led to a considerable dissection of the deposits just mentioned. In describing in more detail those parts of Vermont and Quebec with which we are concerned, it is convenient to divide the area on a topographic basis into an eastern highland, which includes part of the Green Mountain Highland; and a western lowland, which includes the southern part of what may be called the Champlain Lowland (Plate xxi, figure a).

THE HIGHLAND.-The highest parts of the main ridge of the Green Mountains are in the highland part of the area. Several points on the ridge are over 4,000 feet in altitude. Mt. Mansfield, which occupies a central position in the area studied, rises 4,364 feet above the sea. Through this ridge and lower ridges on the east and west, the three largest west-flowing rivers of Vermont have cut deep water-gaps. The depth to which these gorges were cut before they were partly filled with glacial materials is not precisely known, for the glacial materials have not yet been sufficiently removed or explored. South of Mt. Mansfield where the Winooski River has cut through the ridge, the gravelly stream bed is less than 330 feet above the sea. North of this mountain the bed of the Lamoille River is

scarcely 150 feet higher.

Several of the longitudinal subsequent valleys east of the main ridge are but little higher than the water-gaps through which they are drained. For example, the valley next east of Mt. Mansfield, the northern part of which drains to the Lamoille River, and the southern part to the Winooski River, is only 740 feet in altitude at the divide on the floor of the valley, between these streams.

The northern ends of some of the longitudinal valleys northeast of the one just described drain northward from similar low divides into Lake Memphremagog, thence into the St. Lawrence River; some

of the valleys southeast drain southward through branches of the White River, into the Connecticut River.

THE LOWLAND.—The lowland part of the area studied is bounded on the east by the high ridge of the Green Mountains, and on the west by the Adirondack Mountains; at the south it narrows and merges into lowlands of a similar character in the middle Hudson valley; at the north it is continuous with the thousands of square miles of almost dead level clay plains south of the St. Lawrence River. Above the plains in Quebec several igneous stocks rise. Brome Mountain and Shefford Mountain are two such stocks situated about 25 miles north of Vermont. Below the level of the plains there is an area extending the entire length of northwestern Vermont which was not completely filled to the general level by glacial deposits. This area is occupied by Lake Champlain. The surface of the lake is 96 feet above the sea, according to Gannett's (1906) dictionary of altitudes.

THE SHORE-LINES.

PROBLEMS STATED.-The problems in mind when this study was begun may be stated as follows:

1. What evidences are there of abandoned Pleistocene shore-lines on the eastern side of the Lake Champlain drainage basin?

2. Are such shore-line features as may be found associated with local bodies of water, or may they be correlated with shore-lines already made out in the western part of the Champlain district?

3. Did any of the Pleistocene lakes which once occupied valleys now draining westward into Lake Champlain drain eastward into the Connecticut River?

PREVIOUS STUDIES.-C. H. Hitchcock and others ('61, p. 93-191) early mapped and described many terraces and so-called shore-lines of northwestern Vermont. Later, Baldwin ('94) studied some of the evidences of submergence along the eastern shore of Lake Champlain, and Chalmers ('98, p. 12-19) makes reference to beaches in southeastern Quebec. Woodworth (1905) has brought together the results of his own observations and those of others in the Lake Champlain district. I give here a summary of the history of the body of water which occupied the valley of Lake Champlain during the retreat of the Wisconsin ice-sheet, as such history has been sketched by Woodworth.

While the southern end of the ice-tongue which occupied the Cham

plain valley stood in the vicinity of the present divide between the Champlain and Hudson valleys, a body of fresh water known as Lake Albany bordered the ice-tongue and drained southward. At a later time the waters of the southern part of Lake Albany were drained away, but the waters of the northern part were held in at a lower level than the original level by a barrier across the basin near Schuylerville, N. Y. This smaller lake has been called Lake Vermont, or Glacial Lake Champlain, for from this beginning it continued to extend northward across Vermont in the Champlain valley as the ice-front retreated. The highest level of this lake was determined by an outlet just east of Quaker Springs, N. Y. This stage of Lake Vermont, Woodworth (according to a verbal statement to the writer) would now call the Upper Coveville stage. A rather gradual lowering of this lake took place until an outlet near Coveville, N. Y., at a level 100 feet lower, inaugurated a period of nearly constant level, known as the Coveville stage. The ice-front now stood somewhere between Port Kent and Street Road, N. Y.1 (opposite the southern third of the Vermont area which is under consideration).

After the lake stood for some time at this level, during which time the ice-front was continuing to retreat, another outlet, through the valley of Wood Creek, took the drainage of the lake and lowered the lake-level another 100 feet. This is the lowest outlet which has been discovered for Lake Vermont. The probabilities are that the subsequent lowering of the lake-level was caused by the leaking out of water toward the north, around or beneath the ice. When the ice no longer formed a barrier across the northern end of the Champlain valley the sea had free access to the present site of Lake Champlain, owing to the fact that the land was depressed at the north. The amount of depression at the site of the present foot of Lake Champlain was about 450 feet. This depression was of the nature of a tilting, for the head of Lake Champlain was not then below sea-level. Since the sea first came into the valley there has been uplifting at the north so that the shore-lines developed at that marine stage are now inclined toward the south at the rate of about 3.65 feet per mile.2 This tilted plane is the upper marine limit.

All the shore-lines made at the different stages of Lake Vermont participated in this upwarping, so they also slope southward.

1Woodworth, 1905, p. 196.

"This figure was obtained from calculation based on the profile, Plate 28, of Woodworth's report (1905, p. 226).