Columnar jointing cuts a traprock cliff near the Hartford reservoirs. Talus is forming at the base of the cliff as the columns break off the rock face. (Note hat for scale).

As the magma cooled into traprock, it also hardened into a tight structure of interlocking crystals. As a result, traprock resists erosion much better than brownstone, and forms high ranges throughout the Central Valley known as traprock ridges. The longest of these, the Metacomet Ridge, formed from traprock lava flows that cooled into rock, were later buried under brownstone, and subsequently reexposed by erosion. All the others formed from traprock that cooled underground, and, thanks to erosion, now are seeing the light of day for the first time. Watching over bustling New Haven are East Rock and West Rock, the city's traprock sentinels. (The tunnel on the Wilbur Cross Parkway is blasted through West Rock.) In Hamden lies the aptly named mass of Sleeping Giant, a traprock colossus sacked out forever in the Central Valley brownstone. To the north in Granby, East Granby, and Canton, a line of small traprock ridges - Manitook Mountain, the Barndoor Hills, The Hedgehog, The Sugarloaf, and Onion Mountain - runs along the western edge of the Central Valley.

Most of the ridges have the same general profile - a steep cliff on the west side and a gradual slope to the east. The west-facing cliffs rise out of a jumble of fallen rock at the base known as talus; little or no talus is found on the gentler eastern slopes. The west-facing cliffs are the most distinctive feature of a traprock ridge. In places where the slopes are too steep to support vegetation, the cliffs jut out of the forest as colorful red- orange masses of rock. New Haven's East Rock and West Rock are especially colorful, and l7th-century Dutch explorer Adriaen Block named the region Rodeberg, meaning "Red Mountain. "'

The Metacomet Ridge traprock barrier was finally breached by the New York, New Haven, and Hartford Railroad in 1839.' The railroad closely followed the path of the Berlin Turnpike, an early toll road for horses and carriages. In the 20th century, construction of Route 15 and Interstates 84 and 91 provided three additional swift passages through the ridge. Modern transportation methods are so effective that the topographic isolation of Hartford from New Haven caused by the Metacomet Ridge is now all but forgotten. However, the path of Interstate 91 through the Metacomet Ridge is still well known to motorists as the steep grade between Exits 18 and 19 where the trucks

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