The Geology of Mt. Mansfield State Forest

by Robert A. Christman

DESCRIPTIONS OF INDIVIDUAL LOCALITIES

Spruce Peak and Sterling Pond

Spruce Peak is the mountain on the east side of Smugglers Notch. It may be reached by several trails or the ski lift. An excellent view of Smugglers Notch and the surrounding country is obtained from the summit. Near the summit along the access road to the ski lift, unweathered mica-albite-quartz schist is exposed and the folding can be clearly seen.

Sterling Pond lies to the northeast of Spruce Peak in a shallow depression along the divide of the mountain. This location is anomalous for a pond because the amount of higher land around it is so small that only a limited amount of drainage area is available for the accumulation of rain and snow. Yet, the outflow of water is almost continuous during the summer. The basin occupied by the pond was probably scoured out by the glacier.

The Sterling Pond area contains talc deposits which probably would be commercial if they were not so inaccessible for mining. These deposits have been studied by A. H. Chidester of the United States Geological Survey and a report is obtainable from the U. S. Government. The occurrence of the talc, as mapped by Chidester, is shown in the sketch of [Figure 8].

Talc has the composition of Mg₃Si₄O₁₀(OH)₂ and is a soft flaky white mineral. The talc-bearing rocks in this area are white, light gray, or light green and usually are irregularly stained yellow-brown by the weathering of an iron-bearing mineral that occurs with the talc. Because of its extremely soft nature, the talc does not form prominent rock exposures. As shown by the figure, the talc can be most easily observed along the shore of Sterling Pond, south of the Green Mountain Club cabin, or along the trail to Smugglers Notch near the top of the first rise from the pond. Here the talc is exposed in the trail as low, rounded, slippery knobs of “messy looking” yellow-brown rock.

As part of the pond is underlain by talc, it is probable that the softness of the talc was a factor in the differential erosion of the basin by the continental glacier.

The talc is believed to have originated by the alteration of a body of ultramafic igneous rock, which is characterized by having a low silica content and a high magnesium content. Sometime during the mountain-building period, the ultramafic igneous rock invaded the pre-existing rocks from an unknown source within the earth’s crust. It is believed that at a later date the original minerals in the igneous rock were altered to talc and other minerals by the action of hot ascending solutions composed principally of water.

Figure 8. Sketchmap showing the location of the talc-bearing rocks at Sterling Pond.

MAP OF THE STERLING POND AREA FOREST BOUNDARY Green Mountain Club cabin STERLING POND To Smugglers Notch EXPLANATION Talc-bearing rock Solid areas are positions of exposures Postulated boundary of talc body beneath pond Trail To Spruce Peak From report by A. H. Chidester

The Long Trail passes by Sterling Pond, where the Green Mountain Club has erected a small cabin overlooking the pond. [Figure 9] is the picturesque view of Mount Mansfield taken from the cabin.

Smugglers Notch

All of the rocks exposed at Smugglers Notch ([Figure 10]) are the mica-albite-quartz schist which locally contains garnet. The large rock boulders in the Notch were derived from the cliffs forming the walls of the Notch. The gradual processes of weathering and breaking up by freezing cause large slabs of the rock to become loose and eventually break off the cliff faces to careen down the mountainside to the valley below, just as King Rock did within historical time. At the north end of the Notch, the large accumulations of such boulders form a talus slope, the name for rock accumulations at the base of a cliff. The irregular stacking of these rocks have formed Smugglers Cave. Smaller openings extend further back under the “rock pile” to where the ground is considerably colder and where the heavier cold air has sunk. Drafts of this air escaping at the foot of the talus is noticeably cool.

The Smuggler’s Face, Elephants Head, the Singing Bird, and the Hunter and His Dog are freaks of nature and a product of man’s imagination. Their existence is due to the haphazard nature in which the rocks on the cliff faces have broken along joint surfaces.

The origin of the Notch is not completely known. The steep walls and the narrowness of the Notch suggest that it could not have been formed by the headward erosion of two streams or by glacial action. It seems most likely that it was formed by the erosive action of an ancient river that once flowed through the area. Because of the high elevation, the only time when such a river could have existed is when the Champlain Valley was filled with ice on the west side of Mount Mansfield, so that the normal drainage of water to the west was blocked by the ice. It is possible that the conditions were such that the water supplied by the melting glacier could only drain to the south through the Notch. After the ice retreated the drainage system was abandoned in favor of lower outlets and eventually the drainage was developed to Lake Champlain in the west. The Notch was modified by the headward erosion of the present small streams. Except for the shape of the Notch, the only evidence for this hypothesis is the occurrence of a weakly-bedded, well-sorted deposit of sand at the north end of the Notch at an elevation of 2050 feet. Such a deposit of sand is characteristic of standing water, which occurring at this elevation indicates that some vastly different drainage system must have existed in the past.

Figure 9. The Chin and Bear Head on Mount Mansfield viewed from Sterling Pond.

Figure 10. View of Smugglers Notch and Spruce Peak from ski slope on Mount Mansfield.

Big Spring

On the south side of the Smugglers Notch road at an elevation of about 1800 feet is the Big Spring which furnishes a tremendous output of cold spring water. The source of the water is not known although it is likely that, like most springs, it is derived from an underground drainage system. The spring probably originates by the seepage of water derived from the winter snows and rainfall in Spruce Mountain through a joint system within the mountain. No buried stream channels could be located between the spring and the base of the massive overhead cliffs. The belief that the spring is related to Sterling Pond is unfounded. It is unlikely that surplus water could be drained from Sterling Pond because it already maintains a delicate balance between the supply of water from rain and snow and the output to the stream flowing to the north.

Mount Mansfield

The summit of Mount Mansfield when viewed from a distance has the resemblance of a face with an exaggerated distance between the nose and upper lips. Accordingly, these peaks are named, from south to north, the Forehead, Nose, Upper Lip, Lower Lip, Chin, and Adam’s Apple. All these points are readily accessible by the Long Trail and the area of the Nose may be reached by the Toll Road or the Ski Lift. From all the points along the crest an excellent view may be obtained. On clear days Lake Champlain and the Adirondack Mountains in New York State may be seen to the west, and Mount Washington in New Hampshire may be seen to the east.

Of the local structures, good exposures of the mica-albite-quartz schist occur along the crest with magnetite and garnet locally abundant. The rocks on Sunset Ridge, which extends west from the Chin, can be seen clearly to be dipping at gentle angles to the west.

The Chin has an elevation of 4393 feet, which is the highest point in Vermont. Most of the schist in this area is nearly horizontal or dipping slightly to the west. However, minor folds are present everywhere and an average reading is difficult to obtain. At the summit much of the schist contains large black grains of magnetite. The summit is reached by the Long Trail along the crest of the mountain from the Toll Road, by the Long Trail from Barnes Camp via Taft Lodge, or from the west by the trail up Sunset Ridge. An excellent view of the Lake of the Clouds and the Adam’s Apple is obtained a short distance north of the summit of the Chin.

Figure 11. “Subway” formed by separation of the rocks along a joint on west slope of Mount Mansfield.

Lake of the Clouds which lies north of the Adam’s Apple and Bear Pond which lies north of Bear Head Mountain are both small shallow bodies of water. The slight depressions in which they occur were probably scooped out by the erosive action of the glacier.

Figure 12. Cave of the Winds. Mount Mansfield, as seen from the east slope of the Chin. The Mount Mansfield Hotel and the Nose are in the distance. The rocks in the foreground show the beginning stages of downhill slippage of a large mass of rock away from the joint.

Between the Chin and Mount Mansfield Hotel are a number of interesting trails. The Subway and Canyon trail on the west side of the mountain follow, for part of the way, joints in the rock which have been enlarged by the downhill slippage of the western block. [Figure 11] shows the nature of one of these passageways. On the east side of the mountain, the Cliff trail passes through a similar joint called “Wall Street.” The Cave of the Winds, reached by a trail just north of the Lower Lip, has formed along another north-south joint. The block which has moved downhill has tilted into the mountain and rubble has filled the gap at the top to form the cave. [Figure 12] shows the appearance of the cave as seen from the eastern slope of the Chin. This photograph shows smaller joints on the Chin which have just begun to be enlarged by slippage of the downhill block. It is possible that some of these open joints date back and partially owe their origin to glacial erosive action.

Drift Rock which is located along the Long Trail south of the Upper Lip has already been described as an erratic boulder moved by the glacier. The glacial striae may be seen in the bedrock northwest of the boulders. Garnet crystals are very abundant in these boulders and specimens of the small red garnets may be obtained here.

The Nose is easily reached along the trail from Mount Mansfield Hotel. From the summit the view is excellent to the south towards Camels Hump, which is one of the prominent peaks of the Green Mountain range. The mica-albite-quartz schist on top of the Nose has many small folds and crenulations.

South of the Forehead along the Long Trail, cliffs of an albite-rich variety of the mica-albite-quartz schist form obstacles which have been surmounted cleverly by the Green Mountain trail-markers.

In the southern part of Mount Mansfield State Forest the Long Trail passes through Nebraska Notch in the vicinity of Taylor Lodge. This notch also was formed by the erosive action of an ancient river that flowed across the mountain at this point, but which has long since been abandoned.

The cliffs at Smugglers Notch as seen looking south from the west wall of the Notch.