Origins of the Traprock Ridges

The extensive ridgetop hiking trails in central Connecticut are fairly well known, with their fine views, blueberries, and sunflowers, e.g the trails on East Rock, West Rock, Mount Higby, the Hanging Hills,  Cathole Mountain, and Ragged Mountain.  However, remarkably few people who live here realize that the Metacomet and other ridges are  of volcanic origin.  Why isn’t this  part of every high school earth science  class? The geologic processes described below have created a rugged landscape with mineral-rich, subacidic soil,  steep talus slopes (fields of basalt chunks), cliffs, and exposed rock outcrops;  these are all habitats for unusual flora and fauna, including rare species – another fact that is not widely known.

View from the southwest of Cathole Mountain (south end of the ridge, in  Meriden)

Connecticut does not have cone-shaped mountains that once rumbled and spewed ash and lava. Instead the lava oozed more slowly from deep, elongated cracks, that started forming 200,000 years ago, when  the supercontinent, Pangaia, began to pull apart.  Tension between the freshly separated continents caused two elongated cracks (faults) to form. The land settled between two deepening faults, creating a rift valley.  Molten lava oozed up through the deepest cracks and spread across the valley, and then cooled and hardened into traprock (basalt). Three separate periods of lava flows formed three beds of variable thickness.  The middle bed (Holyoke Basalt) may be hundreds of feet thick.

The valley gradually filled with sediment eroded from what used to be high mountains in eastern and western Connecticut. The eastern and western highlands are still many hundreds of feet higher in elevation than the lowlands of the Connecticut valley.  Each successive bed of basalt (cooled lava) was buried by sediment that was compressed into a reddish-brown sedimentary rock, known as brownstone or New Haven Arkose. Total deposition was two miles thick at the Eastern Border Fault in Middletown.  Climate conditions at that time were tropical, which accounts for the red, oxidized color of the sedimentary rock and its low mineral content.[1]

Because the rift valley was still deepening along the Eastern Border Fault (often called the “trapdoor”), the rock beds all tilted down to the east, by 15 to 25 degrees.  The broad basalt beds were glued together by sedimentary brownstone.  Eventually the beds  broke apart into multiple, tilted  “sandwich” chunks.  Over time, especially during the periods of glaciation, the  process of erosion exposed the higher, western, “up-tipped” edges of the these basalt beds, since  traprock is considerably harder than brownstone. The sedimentary“glue” weathered away between the layers of basalt rock. The western edge of each broad basalt slab became a basalt ridge.  A traprock ridge  typically has cliffs and steep talus fields on the west side, and a gently sloping eastern slope,  corresponding to the original easterly tilt of the rock formation.

 The ridges often show an interesting triplet pattern: a central taller ridge corresponding to the thick bed of Holyoke Basalt, is flanked by two much lower parallel ridges, corresponding to the thin slabs of Talcott and Hampden Basalts).  The far north end of Cathole Ridge shows this pattern very clearly.

Most of our  traprock ridges originated as described above, from the western edges of cracked, tilted  lava slabs. However, some “intrusive” formations like Sleeping Giant and West Rock[2] were formed underground. The oozing lava cooled slowly underground, rather than on the surface. In this slow-cooled rock, called diabase, crystals are larger and visible to the naked eye. The  rock weathers more slowly, but mineral composition is identical to basalt. These intrusive ridges were buried by sediment and then exposed by weathering and glacial scour, just like the basalt ridges.

Central Connecticut’s  continuous, above-ground traprock ridge system extends northerly into Massachusetts, but cracks in the rift valley oozed lava as far north as Newfoundland.  Intermittently exposed basalt also occurs in Newark and Hoboken, New Jersey (the Palisades) and further west in the Pomperaug valley in Southbury and Woodbury, Connecticut.

The sub-acidic, volcanic soil on traprock ridges is fine-textured and less acid and richer in minerals, like calcium, magnesium, and potassium- than soil derived from brownstone, granite, gneiss or schist.  Second, the thin soil, rocky outcrops, talus,  boulder fields, and steep slopes are specialized ecological conditions quite different than that found in lowland forest, and are associated with a unique suite of plants and animals.

Partly open glades are characteristic, The “lawn” is actually Pennsylvania sedge, which remains low, naturally. A characteristic suite of plants (besides Penn Sedge) grows in this habitat.

These ecological communities have been designated  as high priority “critical habitats”  by CTDEEP.  However, like  the low public awareness of the ridges’ unusual volcanic origins, public awareness of the traprock ridges’ high conservation value seems surprisingly low, even in the towns where the ridges are major landscape features, like Meriden, Berlin, Cheshire, and New Haven.


[1] An infertile “tropical” soil with a reddish color forms from brownstone parent material unless it is enriched by basalt glacial till or by river sediment.   The well weathered soils of the tropics are also known for their red color.

[2] The name of West Rock (in West Haven and Hamden) changes moving northward, first to Prospect Ridge,  and then to Peck Mountain in North Cheshire.

Photos of the five critical traprock habitats  may be viewed via links to public facebook albums (Sigrun Gadwa).

  • album1    West Peak Summit in September – CT Botanical Society Excursion
  • album2    Traprock Ridge Natural Resources of Central Connecticut
  • album 3   Wetlands Below Traprock Ridges
  • album 4   North end of Mount Lamentation

Symphiotrichum laevis is a spectacular, densely flowering, dry-site aster found on traprock summits.



About sigrungadwa

Consulting Ecologist and Registered Soil Scientist. My firm, Carya Ecological Services, LLC conducts inventories of vegetation, wetlands, and habitat; wetland delineations; and ecological assessments - for open space grants, application reviews, and applications for open space/cluster developments. We work either as the lead firm or as a subconsultant. I have a Masters Degree from UCONN-Storrs, Dept of Ecology & Evolutionary Biology (1997) & a Bachelors from Brown U. in Biology.
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