Open Space and Recreation Plan - Town of North Andover

T O W N O F N O R T H A N D O V E R
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4.0 NATURAL RESOURCE INVENTORY AND ANALYSIS
4.1 GEOLOGY
Surficial Geology
The North Andover landscape shows the unmistakable influence of continental glaciations. This is most notable
in the presence of a number of oval-shaped hills, as well as many irregularly shaped swamps and bogs resulting
from the disrupted drainage patterns. “These hills are known as drumlins. They are oval shaped hills made of a
jumbled combination of clay, sand and boulders called till. The till can be recognized by the mixed distribution
of boulders, pebbles, and clay. Till is very hard and generally gray in color. The soil at the top of the till where
it has been weathered is typically a brownish color. Till is sediment deposited directly by the glacier" (Roberts,
1977).
There are a dozen large hills in North Andover. Roberts identifies two (2) drumlins, but one, Claypit Hill, may
be a kame. ). A kame is the remnant of deposits of a glacial stream that formed on the valley wall while the
glacier filled the valley. Claypit Hill shows deposits of "fine sand to gravel to rocks of several inches in diameter.
One fresh cut in the hillside showed well defined layering of gravel" (Doucette, 1990). Perhaps this is an
ancient kame reshaped by a later glacier into a drumlin.
Much of the southern and southeastern portion of town is made up of ground moraine, "bedrock covered by a
thin sheet of till" (Roberts, 1977). The combination of poorly drained clay-rich till and the disruption of
drainage patterns mentioned above has resulted in the many swamps, marshes and bogs found throughout
Town, particularly in the southern and southeastern portion. These poorly drained soils and broad wetlands
present serious limitations for development (particularly with the lack of public sewer), but do provide
opportunities for protecting open spaces.
Bedrock Geology
Two major bedrock zones underlie North Andover; the Merrimack Belt traverse along the extreme northern
portion of Town, and the Nashoba Zone underlies the remainder (Zen et al, 1983). The Merrimack Belt is
mostly composed of metamorphosed sandstone and siltstone. The Nashoba Zone is primarily dominated by
Andover granite and Sharpners Pond Diorite.
These two major bedrock zones are split by the Clinton-Newbury fault that runs southwest to northeast just
north of Lake Cochichewick. This major structural feature runs through southern New England and was most
active over 250 million years ago (Skehan and Murray, 1980). There have been minor earthquakes associated
with this fault including a quake measuring 2.5 on the Richter scale that was felt in Littleton, MA in 1990
(Cook, 1990). A second fault, the Assabet river fault, may run through Town in a north / south direction
along the eastern edge of the main mass of Andover granite (Goldsmith, 1991).
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