_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.
_Erratics_ are boulders moved and deposited by glaciers. Often these
boulders are found in environments foreign to them. On the high parts of
Mount Mansfield, a few boulders other than the mica-albite-quartz schist
are found which have reached their present position by glacial
transportation. A particularly interesting erratic is Drift Rock. One
may note by referring again to Figure 7 that the glacial striae pass
beneath the large boulders indicating that the boulders were not at
their present position when the striae were formed. Thus, these boulders
are aptly named for they must be considered erratics even though their
composition is the same as the surrounding rocks. As a logical
speculation it is probable that they were plucked out of the bedrock
just a short distance to the northwest of their present position and it
is possible that these were the very boulders that formed the striae.
Other evidence of glaciation is the presence of _cirques_ which are the
high mountain basins in which mountain glaciers originate. Small
mountain glaciers probably existed on Mount Mansfield, particularly in
the east side of the mountain where the topography has the suggestive
form of cirque-like walls and basins.
Melting of a continental glacier causes deposition of the sand and
gravel scooped up and transported by the glacier. Some of this material
is plastered beneath the moving glacier, some is simply let down as a
blanket and terminal accumulations, and some is transported by streams
of melt water to form deposits in which the sands and gravels are
somewhat sorted in size by the action of the water.
[Illustration: Figure 7. Drift Rock, glacial erratics along the Long
Trail on Mount Mansfield. Note the glacial striae trending away from
the viewer and passing beneath the boulders.]
The blanket type of glacial deposits probably were present over much of
the Mount Mansfield area at one time, but have been removed from the
upper mountain slopes by erosion. Such deposits are present at lower
elevations but are only rarely discernible because of the heavy
vegetation and soil cover. Sand and gravel deposits derived from
post-glacial streams are located in some of the lower valleys as, for
instance, along the highway north of Barnes Camp.
North of the old road and northeast of the camping area along one of the
main streams on the west side of Mount Mansfield, fine-grained, sandy
lake deposits occur at an elevation of about 1900 feet. The nature of
the deposits indicates that they were formed in ponds marginal to the
ice sheet when the continental glacier occupied the Champlain Valley but
did not extend over the crest of Mount Mansfield.