The Traprock Landscapes of New England. Peter M. LeTourneau

Rock, Red Rock, Blue Hills); and tales and legends (Mount Lamentation, King Philip Mountain).

      Rising to an impressive elevation of 1,024 feet, West Peak (background) is the highest traprock summit in the Connecticut Valley south of Mount Tom in Massachusetts, and the tallest peak located within twenty-five miles of the Atlantic coast south of Maine. View from Ragged Mountain.

      Illuminated by evening alpenglow, Ragged Mountain’s sheer cliffs project an alpine majesty lacking in many of the taller, but more rounded, mountains of New England.

      In the legends of the Quinnipiac River tribes, Sleeping Giant (Mount Carmel) ridge was the reclining form of the giant, Hobbomock, put to sleep for all time for his mischievous deeds. Today, the diabase ridge is part of popular Sleeping Giant State Park, where dozens of hiking trails, a stone lookout tower, and picnic facilities await outdoor enthusiasts.

      The “blue hills” of the Western Range stand high above the misty Quinnipiac River lowland. From north to south along the horizon (right to left), Peck Mountain, Mount Sanford, High Rock, and the north end of West Rock Ridge form a line of lesser-known traprock peaks on the southwestern edge of the Connecticut Valley.

      The two main trends of traprock hills in the Connecticut Valley result from slight differences in their geology. The Metacomet Mountains, from Saltonstall Ridge to Mount Holyoke, are made up of basalt, which is finegrained lava that cooled quickly at or near the surface. The Western Range, from East Rock to Manitook Mountain, formed from basaltic magma that cooled slowly beneath the surface in dikes and sills, creating a coarse-grained igneous rock called diabase.

      The traprock hills follow two major trends through the Connecticut Valley. The main range forms a great “wall” of crags from Mount Holyoke in the north to Saltonstall Ridge near Long Island Sound. Benjamin Silliman described the particular linear arrangement of this range in 1820: “In many parts of this district, the country seems divided by stupendous walls, and the eye ranges along, league after league, without perceiving an avenue, or a place of egress.” This main range of lava flows are herein designated the Metacomet Mountains, to distinguish their particular topographic aspect, and to streamline the cumbersome geographic nomenclature of the Connecticut Valley (please refer to Note on Terminology and Usage).

      The volcanic veins and dikes that follow the western side of the valley as a series of discontinuous ridges and hills were named the Western Range by the noted geographer William Morris Davis in 1898. The Western Range runs from West Rock near New Haven to Manitook Mountain in Suffield, Connecticut, near the Massachusetts border. The specific rock type (diabase, see below) and characteristic “massif” landforms distinguish the Western Range from the typical cuesta ridges of the Metacomet Mountains. Known mainly to local hikers, the remote summits of the Western Range are among the least disturbed areas in the Connecticut Valley. The hills of the Western Range also have the most colorful names of any of the Connecticut Valley traprock crags, including Sleeping Giant, Onion Mountain, the Hedgehog, and the Barndoor Hills. Among the most scenic landforms in the region, East Barndoor Hill (el. 580 ft.) and West Barndoor Hill (el. 640 ft.) in West Granby, Connecticut, form a particularly interesting pair of steep traprock promontories separated by an unusually deep and shady notch.

      Moonrise over the cliffs of South Mountain. Basalt is the most abundant crustal rock type on both the earth and the moon. The lunar “seas” were formed from enormous outpourings of basalt lava roughly 3 billion years ago. Radiometric ages determined from the samples of the CAMP basalts indicate that the lava flows of the Connecticut Valley are approximately 201 million years old.

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      Born of Fire

      TRAPROCK GEOLOGY

      Eruptions … took place from great fissures … of the earth’s crust. The lava which then came up and filled the fissures, and in many places outflowed, is the rock we now call trap.

      James D. Dana, On the Four Rocks of the New Haven Region (1898)

      The story of the Connecticut Valley begins in the early Mesozoic era (the age of dinosaurs), a time when the continents were joined in a single large landmass, or supercontinent, called Pangaea (“all land”). During the Late Triassic period, beginning about 220 million years ago, global tectonic forces caused the crust of Pangaea to thin and separate, a precursor to the eventual formation of the early Atlantic Ocean in the Middle Jurassic, around 160 million years ago.

      The thinning and fracturing, or “rifting,” of the continental crust was accompanied, and in part caused by, an enormous plume of basalt magma rising from the upper mantle and lower crust (melted rock beneath the surface is called magma; lava is magma that cooled at or near the earth’s surface).

      Approximately 200 million years ago the magma plume reached the surface, and voluminous quantities of lava erupted through cracks and fissures, flooding huge areas of the proto–North Atlantic region with basalt. The low-viscosity (very fluid) lava produced by the fissure volcanoes formed great “lakes” of lava in the low-lying rift valleys. Most of the traprock ridges and hills that dominate the landscape of the Connecticut Valley are the tilted and eroded remnants of these enormous lava flows.

      Some of the magma did not make it to the surface, but instead cooled in cracks and veins forming dikes (vertically oriented bodies) and sills (roughly flat-lying layers or lenses). The entire Western Range, from East Rock in New Haven to Manitook Mountain in Suffield, including Mount Carmel in Hamden, are intrusive dikes and sills that have been exposed by erosion of the less resistant surrounding sedimentary rocks. A large number of dikes and sills also occur in East Haven, North Haven, and North Branford in the southeastern part of the Connecticut Valley.

      Other large-scale dikes, including feeders for the lava flows, cut through the older Paleozoic-age rocks outside the Connecticut Valley, and in some cases form linear features that can be traced for hundreds of miles across the landscapes of eastern North America, the Iberian Peninsula, northwest Africa, and northeastern South America. On large-scale maps, these dikes form a distinct radial pattern pointing toward the main magma plume, or “hot spot,” located near the southeastern U.S. Atlantic margin.

      The volcanic rocks of the Connecticut Valley are part of one of the largest terrestrial eruptions of basalt in the geologic record—only the extensive lava flows of the Columbia River plateau in the northwestern United States, the Siberian Traps, or India’s Deccan Traps are comparable. Volcanic rocks correlating in age with the Connecticut Valley basalts are found in South America, North Africa, and western Europe; for convenience this entire suite of Triassic-Jurassic-age volcanic rocks is termed the central Atlantic magmatic province, or CAMP.

      In eastern North America the CAMP basalts form picturesque landforms such as Mount Pony near Culpeper, Virginia; the Watchung Mountains in New Jersey; the Palisades cliffs along the Hudson River; the Orenaug Hills in the Southbury-Woodbury area of Connecticut; the Pocumtuck Hills of the Deerfield Valley; and North Mountain on the Bay of Fundy. The vast quantities of carbon dioxide, sulfur dioxide, and volcanic ash that accompanied the CAMP eruptions are implicated in global climate change and disruptions of the biosphere at the Triassic-Jurassic boundary that, in part, led to the rise of the dinosaurs in the Early Jurassic period.

      The supercontinent of Pangaea during the Early Jurassic, about 200 million years ago. The traprock hills of the Connecticut Valley belong to the