What 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.
They’re also among our oldest. 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. 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.
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Though the two rocky ridges that bookend our city appear distinct, they’re actually connected. Picture East Rock and West Rock as the top and bottom of an I beam lying on its side, Evans suggests. 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.
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. This area was later blasted open to build Whitney Avenue, but evidence of the beam remains on the west side of the road. 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.
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.
Evans 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.
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. 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. 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.”
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. Here you can see that the entire I beam has tilted eastward. Slanted strips of rock along the cliff face have broken into regular hexagonal columns. 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. These same 120-degree hexagonal shapes can be found in geological formations in Wyoming, northern California and Ireland’s famous Giant’s Causeway.
And what about that beautiful reddish glow of the cliffs late in the day? The color may make the rock look like sandstone, but it’s not, although sandstone can be found in small amounts at the base of the cliff and in some pockets on the upper side. Evans chips at the surface of a rocky column to reveal its natural dark gray color underneath. The red comes from iron that was oxidized inside cracks in the stone—the very cracks along which it later broke apart—exposing what is essentially rust, Evans says. A small amount of oxidation may have occurred since the cracking as well.
The volcanoes that created these impressive rock faces left evidence behind in modern-day western Africa and Brazil as well, “so collectively… this magma province is one of the largest that the earth has ever produced,” Evans says. “It’s one of the world’s largest super-duper volcanoes.” But volcanoes are just half of the East Rock-West Rock story that Evans describes as a “drama in two acts, separated by an intermission that’s 200 million years long.” Tomorrow, come back for Act Two, as seen from West Rock.
East Rock Park
Entrance near Davis St and Farnam Dr, Hamden (map)
www.newhavenct.gov/…east_rock.htm
Written by Kathy Leonard Czepiel. Photographed by Dan Mims. Images depict stretches of East Rock cliff face. This story originally published on October 25, 2018.
