Surficial Geology of the Danville/St. Johnsbury
by George M. Haselton and George Springston, 2000
Photos by George Springston unless otherwise credited.
||Photo 1 View from Coles Pond Road looking northeastward
across the Sleepers River valley and the rolling hills of Danville.
Burke Mountain is the prominent peak on the skyline. All of these hills
were overridden by the Laurentide ice sheet during the Wisconsinan glaciation.
The Laurentide ice sheet. Our region was entirely
covered by a large continental glacier known as the Laurentide ice sheet,
which reached its maximum size about 24,000 years ago. This was the
most recent of several ice sheets that spread over the area during the
Pleistocene Epoch (about 1.8 million to 10,000 years ago). It stretched
from Long Island back to its source areas in central Quebec and the
eastern Hudson Bay Lowland and it buried the highest peaks of the Green
Mountains of Vermont, the Adirondacks of New York, and the White Mountains
of New Hampshire.
When the ice was at its maximum extent, world-wide sea level was about
400 feet lower than it is today due to the massive amount of water locked
up in glacial
ice around the world. In glaciated regions the tremendous weight
of the ice depressed the land underneath. In this part of Vermont the
land surface was depressed about 800 feet.
The Laurentide ice sheet slowly retreated across southern Rhode Island
and Connecticut around 21,000 years ago. As the ice melted, worldwide
sea-level quickly rose, and after a few thousand years, the Earth's
crust began rebounding. The ice margin retreated into north-central
Vermont by about 14,000 years ago.
||Photo 2 Glacial Till exposed in the bed of
Roy Brook in Danville. The mattock head is resting in the water of the
brook. The stones in the brook bed to the left of the mattock head are
glacially transported pebbles and cobbles which are actually part of
the glacial till. Those to the right are loose stones which have been
swept along by the current of the stream. The till here is very hard
and is difficult to excavate, even with a mattock.
Glacial till. In the Danville/St. Johnsbury area there are steep,
freshly eroded river banks which expose bluish-gray "clay" with stones
in it. This is actually a variety of glacial till, the unsorted material
left behind when the ice sheet retreated. This particular till is called
basal till. The stones are of all sizes, from sand to huge boulders.
The stones have rounded edges from the abrasion during glacial transport
and many have flat sides (facets) where they were ground down on one
side as they were scraped over the underlying land surface. Many will
also show scratches on them where they were scraped against harder stones
along the way. The matrix material around the stones consists of silt,
clay, and some sand. If you try to dig into it with a shovel you'll
see why a local name for this material is "hardpan". This material is
hard for two reasons: First because the fine silt and clay grains pack
together tightly and second because this material was deposited at the
base of the glacier while the ice was still moving southward over it.
Thus it has been highly compacted by the weight of the overriding ice.
The source areas of glacial till in the Danville/St. Johnsbury area.
The stones in the till give us an indication of the bedrock types that
the glacier has overridden during its journey down from the north. Studies
have shown that glaciers pick up and drop material as they move along.
Although this glacial ice moved from northern Canada, most of the stones
are from less than 10 or 20 miles away, with only a very small fraction
having moved more than 40 miles. Most of the stones found in glacial
till in this area are of the local Waits River Formation, a dark gray
metamorphosed limestone (an impure marble). One way to test for such
lime-bearing rocks is to place a drop of dilute hydrochloric acid on
the sample. If it is limestone or marble it will fizz vigorously. The
remainder of the stones are mostly coarse-grained mica-bearing schist
(another metamorphic rock) and granite (an igneous rock), both also
of local origin. When unusual rocks are encountered in the till it is
sometimes possible to trace them back to their bedrock source areas.
In this part of Vermont such studies indicate that the ice moved in
from the north or northwest.
||Photo 3 Glacial striations on ledges alongside
U.S. Route 2, Danville (Photo by Charles V. Cogbill). This is a view
of a nearly horizontal glacially polished surface on top of a roadside
ledge. The lines which run from top (north) to bottom (south) in the
photo are scratches made by the stones held in the base of the ice sheet
as it moved generally southward over the land. These glacial striations
have a bearing of 184 degrees (almost due south). Fainter striations
with an azimuth of 152 degrees can be seen running diagonally down to
the right in the lower right-hand side of the photo. These striations
cross-cut the 184 degree striations and are thus younger.
Glacial striations are easiest to see when the sun is low. Wetting the
surface of the rock with a little water can make the striations show
up much better.
Glacially polished surfaces.
The surface shown in the photo is
very smooth. The rock has been polished by the fine-grained rock fragments
which make up much of the load of the glacier. Thus, the glacier generally
tends to smooth and polish the surface of the rocks it grinds over.
Glacial striations are caused by the larger stones, which gouge out grooves as they are
scraped over the rock by the moving ice. In this part of Vermont it
is uncommon to see such beautifully polished surfaces because the lime-rich
bedrock weathers quickly. After the retreat of the glacier, this particular
surface was protected by a covering of glacial till, which was removed
when the highway was constructed.
||Photo 4 Varved Lake Deposits in the Passumpsic
River Valley, St. Johnsbury. The photo shows a sedimentary deposit made
up of light-colored layers of silt and dark-colored layers of silty
clay. These layers, called varves, formed in Glacial Lake Hitchcock.
Photo by Charles V. Cogbill.
Varves are annual layers of fine-grained sediment deposited into lakes
in the vicinity of glacial ice. One pair of light and dark layers forms
over the course of each year, the light layer being a relatively coarser-grained
warm season deposit and the dark layer being a relatively finer-grained
winter deposit. These varves formed in Glacial Lake Hitchcock, a lake
which existed in the Connecticut Valley and some of its tributaries
(including the Passumpsic River) at the close of Pleistocene Epoch.
Since varves are annual layers, geologists can count them and determine
how long the lake lasted.
Glacial Lake Hitchcock.
Approximately 18,000 years ago the Laurentide
ice sheet had retreated as far north as New Britain and Rocky Hill in
central Connecticut. As the ice margin retreated further northward up
the Connecticut River valley, glacial meltwater was backed up in the
Connecticut River valley behind a bedrock ridge at New Britain and a
sand and gravel deposit at Rocky Hill, causing the formation of Glacial
. As the glacier retreated northward along the course
of the river, the lake also expanded northward, eventually stretching
all the way into northern Vermont. In the Passumpsic River valley, the
lake deposits extend as far north as Burke and in the Connecticut River
valley they extend up to the vicinity of Littleton, New Hampshire. Other,
somewhat younger lake deposits extend even farther north in the Connecticut
River valley. Lake Hitchcock finally drained approximately 12,500 years
ago. The varved lake deposits of Lake Hitchcock served as the raw material
for the brick-making industry which flourished in the region from the
19th century up through the early 20th century.
Brick-making with Lake Hitchcock clay.
The clay-rich portions
of the lake deposits in the Connecticut and Passumpsic valleys were
the raw material for a very important early industry: brick making.
Clay was dug out from a pit or bank, mixed with a small amount of sand
as a binder, shaped into bricks in wooden forms, and set out to dry.
After partial drying, the bricks were fired in a kiln. Clay is a dense
material and was difficult to move in large quantities in the days before
trucks. Therefore, nineteenth century brick yards tended to be located
on good deposits of lake clays.
Photo 5 Sand and gravel pit excavated along
the crest of the Passumpsic Valley esker in St. Johnsbury. View is looking
An esker is a long, narrow, often winding ridge composed of water-lain
deposits of sand, gravel, and boulders which formed as a stream deposit
in a tunnel within or at the base of a retreating glacier. When the
glacier melts away, the snake-like ridge remains behind.
The Passumpsic Valley esker is one of the longest and finest in Vermont,
if not New England. It extends from St. Johnsbury northward past Lyndonville,
where it splits into two branches: One extending up the valley of the
Sutton River to West Burke and the other extending on up the Passumpsic
River valley to East Haven. In some places it is over 150 feet thick
and two or three hundred feet wide.
|Photo 6 Panorama of the sand and gravel pit
seen in Photo 8. At the left end of the panorama the deposits consist
mostly of outwash sands on the west side of the esker. Another view
of the outwash is seen in Photo 8. Outwash is sand and gravel deposited
by meltwater streams in front of the end moraine or the margin of an
||Photo 7 Closeup view of a fault in gravel in
the core of the Passumpsic Valley esker. A fault is a fracture or break
along which the two sides have moved relative to one another. Although
faults are often thought of as forming in rock, they can also form in
unconsolidated sediments. The fault in this photograph runs from the
upper right to lower left. The fault probably formed as ice melted away
from the sides of the esker, causing slumps and faults in the deposit.
||Photo 8 The sloping layers of sand and gravel
behind the group of geologists are outwash deposits on the west side
of the Passumpsic Valley esker. The layers dip to the west and formed
as glacial meltwater streams carried sediment off of the glacial ice
margin into the upper end of Glacial Lake Hitchcock. Similar deposits
occur on the east side of the esker as well.
||Photo 9. Varved lake deposit on top of the
Passumpsic Valley Esker. This photo was taken directly beneath the Photo
5 site and shows varved lake deposits of silt and silty clay. The varves
are cross-cut by faults which formed when ice on either side of the
esker melted away, resulting in the partial collapse of the esker. The
presence of the varves above the gravels and sands of the esker and
outwash indicates that deposition of sediment in the lake continued
for many years after the margin of the glacier had retreated northward
towards Canada. Photo by Charles V. Cogbill.
||Photo 10 High water at Emerson Falls, Sleepers
River, St. Johnsbury. Here the Sleepers River tumbles down a ledge of
bedrock, having eroded away any surficial deposits left behind by the
||Photo 11 Low water at Emerson Falls. The bold,
resistant bedrock ledge here causes an abrupt change in the gradient
or slope of the river. Such a spot is called a nick point. Until such
time as the stream can erode its way through the resistant rocks which
make up the falls, the bed of the river upstream cannot be lowered below
the nick point.
||Photo 12 Pothole cut into bedrock at Emerson
Falls. The notebook, for scale, is 7.5 by 4.7 inches. Potholes are formed
by the scouring action of stones caught in eddies in the river. When
the water is low, several excellent potholes can be seen.
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