![]() As the magma cooled, it shrank and cracked, and the “cracks organize themselves into a tiling pattern” because, Evans explains, breaking into these regular shapes requires the least amount of energy. Slanted strips of rock along the cliff face have broken into regular hexagonal columns. Here you can see that the entire I beam has tilted eastward. Our next stop is halfway up the road to the East Rock summit, where we pull over to take a closer look at the columns of basalt. In the winter, Evans says, when the trees are bare and snow highlights East Rock, it’s easy to look east from Whitney Avenue and see where the beam goes “straight up the mountain.” The beam rises up here from Whitney and Armory, where Deepwood Drive winds up it, then plunges under Dixwell Avenue before resurfacing again past Pine Rock Avenue and running into West Rock Ridge, all along the fissures of ancient volcanic eruptions. This is the easiest place to see a cross-section of the long beam that connects East Rock to West Rock, but it’s not the only one. There Evans points out the line where the exposed cross-section of the beam meets a flow of sand that was washed down the valley toward the ocean and blocked by this “intrusion.” The sand piled up in “rubbly,” pebbled horizontal layers still visible today as sandstone against the trap rock’s mass.Įvans picks up a couple of pieces of trap rock and breaks them open, then fishes two tiny magnifiers from his pocket so we can see the difference between the stone at the outside edge of the beam-magma that cooled quickly-and stone from the inside of the beam-magma that cooled more slowly and therefore had time to form tiny interlocking crystals. We cross Whitney at the intersection with Armory Street and walk uphill about 100 feet to a rocky outcrop I’ve driven past thousands of times without noticing a thing. This area was later blasted open to build Whitney Avenue, but evidence of the beam remains on the west side of the road. We park in the lot at the Eli Whitney Museum, where Whitney once built his arms factory at a natural waterfall that cascaded over the beam connecting the two ridges. The central piece that once more clearly linked the two ridges can still be spotted in several places across New Haven, most notably at Evans’s favorite geological outcrop in the city, where we head together to take a look. ![]() Picture East Rock and West Rock as the top and bottom of an I beam lying on its side, Evans suggests. Though the two rocky ridges that bookend our city appear distinct, they’re actually connected. The rock we see today rising up as the East and West Rock ridges was once underground magma that cooled and solidified and later rose to the surface. The geological story of how East and West Rock came to be takes us back 200 million years, Yale geology professor David Evans says, to a time when active volcanoes were spewing fountains of lava from fissures in the earth. W hat New Havener hasn’t been stunned on a sunny evening by the view of West Rock’s cliffs bathing Westville in their carmine light, or by East Rock’s sheer rosy face, frilled with trees, basking in the glow of the sinking sun? Holding the city as if between two fingertips, these ridges of trap rock may be our finest natural landmarks. ![]()
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