Natural Resource Inventory and Assessment - Town of Guilford

Section A. GEOLOGY
A.1 Introduction
See separate Map Atlas for map references
The geology of any area is interesting and important primarily because it constitutes the foundation
and context of all other natural resources. The geological resources of Guilford
• offer tangible evidence of cataclysmic events of the distant past
• shape our scenic hillsides, valleys, ridges, outcrops, and coastline
• determine the characteristics of our soils, indirectly determining the flora and fauna that the soil
supports
• determine flow of surface and sub-surface water
• determine location of aquifers
• contribute to water chemistry of freshwater ecosystems
• are major determinants of human settlement patterns and development constraints
• form structures that provide specialized habitats
Aside from historic granite extraction and some small sand and gravel operations, Guilford is not
known for its mineral wealth and supports no active quarries or mines. There are no gemstones (except
isolated occurrences of garnet) and the only semi-precious minerals present are quartz and feldspar.
This section relates the evidence of geologic history to specific sites in Guilford and points out the
interrelationships of geological resources with other natural resources. The Town’s geology, like
Connecticut’s as a whole, is varied and complex, mainly because of its location in an ancient collision zone
of the earth’s tectonic plates. Some understanding of geologic history helps to appreciate both the regional
context and the relationships among Guilford’s varied landscape features. However, as a detailed natural
history is beyond the scope of this document, the reader is encouraged to study one or more of the
excellent text references listed at the end of this section.
A.2 Geographic context
The linear distance between the point of Sachem’s Head, Guilford’s southernmost coastline feature,
and the Durham border on the north, measures about 13 miles. East to west, the Town spans six miles at its
widest. In terms of actual land area, it contains 47 square miles.
Guilford lies almost entirely in the landscape region of Connecticut called the Coastal Slope, a zone
that begins approximately 12 miles north of the coastline. In this zone the plane of hilltop elevation
decreases at a slope of about 50 feet per mile, about twice the slope of zones further inland. The typical
elevation of hilltops near the Town’s northern border is 400 to 500 feet above sea level. The northwest
corner of Guilford is exceptional in that it technically lies outside the Coastal Slope and is part of the
Central Valley landscape region. The Town’s highest hilltop elevation, 720 feet, is located in this zone. This
spot is 12 miles north of the coastline, at the northeastern end of Totoket Mountain, in the area of Bluff
Head. Here Totoket Mountain presents 420 feet of vertical relief relative to the nearby Coginchaug River
valley. The Town center, by comparison, lies just 20 feet above sea level.
The West River valley, Guilford’s largest drainage basin, drops from Lake Quonnipaug (elevation 180
feet) to sea level at a slope averaging 25 feet per mile. (See Section C. Freshwater for a full description of river
basins and other water features.)
Natural Resource Inventory and Assessment Town of Guilford, CT A1
Geology