Figure 6. Three dimensional diagrams showing variations in dip and strike. Plane in A strikes N 45° W and dips 45° SW; B strikes north-south and dips 60° east; and the plane in C strikes N 45° E and dips 30° SE.
Another structural feature of the schists is the breakage of the rocks along definite plane surfaces called joints. These usually occur in systems formed by a number of parallel joints. The joints formed as a result of stress and strain operating on the rocks during periods of mountain-making and vertical uplifts. Information as to the nature of these forces might be obtained if all the joints were carefully recorded and plotted on a map.
On Mount Mansfield some of the prominent topographic features appear to be controlled by joints. Much of the north-facing cliff on the Nose is controlled by a joint trending N. 65° W., and a similar face on the Lower Lip is controlled by a joint trending N. 60° W. Along the crest of Mount Mansfield a number of joints trend about north-south. Joints of this system in the steep cliffs on either side of the crest of Mount Mansfield have been separated further by the tendency of the rocks to creep down slope under the force of gravity. These joints form the canyons or narrow passageways which are traversed by some of the trails. On Maple Ridge at about 3300 foot elevation the trail crosses a joint trending N. 50° E. which is conspicuous for its four-foot width and the extent and the straightness of the break. A number of joints belonging to this system are found along Maple Ridge.
GLACIAL HISTORY OF THE AREA
Introduction
The geologic time division previous to the present one is called the Pleistocene or the “ice age.” During this time, large continental glaciers advanced over the northern part of North America several times. The cause of the ice age is not known with certainty and whether geologic history will repeat itself is a matter of conjecture. However, it is established that these vast ice sheets covered New England and that the last ice sheet melted back from the Mansfield area about 12,000 years ago.
If one stands on the crest of Mount Mansfield and looks westward over the Champlain Valley, it is difficult to visualize this entire valley completely filled with ice of the continental glacier. Yet, the evidence shows that the ice sheet was so thick that it completely covered Mount Mansfield at one time.
Evidences of glaciation
Two types of evidence, glacial striae and erratics, show that Mount Mansfield was over-ridden by the continental glaciation. Striae are scratches in the bedrock which were produced by the sharp edges of rocks protruding from the sole of the moving glacier. These scratches show the direction in which the glacier was moving at a particular spot and the average of many readings gives an accurate value as to the overall direction of movement of the ice sheet. On the Long Trail between the Mount Mansfield Hotel and the Chin on Mount Mansfield, striations may be observed at a number of places. Some of the positions where readings were made are indicated on the map by the triangular-pointed arrows. Faint striae may be seen near the entrance of the Mount Mansfield Hotel and more conspicuous ones are visible on the west side of the roadbed of the secondary road that intersects the Toll Road just below the Hotel. [Figure 7] shows a photograph taken at Drift Rock in which the striae are clearly visible.
The average trend of the striae on Mount Mansfield is about N. 50° W. The movement of the ice is presumed to have been nearly north-south down the Champlain Valley which was deepened by the erosive action of the ice. These facts seem to suggest that the movement of ice over Mount Mansfield was marginal and nearly 45° to the axis of the main ice tongue.