“Rocks are records of events that took place at the time they formed. They are books. They have a different vocabulary, a different alphabet, but you learn how to read them.” -John McPhee

Every day, I see East Rock from my lab’s 6^th floor lounge. This place (East Rock, not my lab) is one of the few interesting places to go to in New Haven. There are nice trails, great picnic spots (my PhD class had a year-end barbecue there; holla Megatrackers!), and a lovely view of all of New Haven and the Long Island Sound. And, of course, the ominous Kline Biology Tower (home to the Molecular, Cellular, and Developmental Biology Department) sticks out like sore thumb. Fortunately, the lab building I'm in is much prettier :) Anyway, enchanted by this geologic feature, I decided to Wiki “East Rock” and was intrigued to learn that it is actually a trap rock ridge. I had never heard such a term before, so I decided to do some more research on it.

View of New Haven from the top of East Rock. Photo credit: Nikhil Bumb

Trap rock ridges, which are known for their rust colored cliffs, are quite common in Connecticut and Massachusetts. Their name comes from the dense, dark, fine grained igneous rock of which they are made. However, “trap rock” is a mining, not geologic, term to describe dark igneous rocks used for road construction. When dark magma originating from the Earth’s mantle extrudes into the crust, it cools slowly and becomes the coarse-grained gabbro rock. However, when the magma gets to the surface, it cools faster, becoming the fine-grained basalt rock that makes up East Rock. Fresh basalt is grey, but after centuries of weathering, the iron in the rock begins to rust, giving the distinctive red color. The weathering also fractures the basalt into the distinctive octagonal and pentagonal columns.

The rust-colored columns that define East Rock.

As hinted before, volcanic activity (from 200 million years ago during the Triassic and Jurassic periods!) and erosion are the two big events that form these ridges. Three major lava flows from unexplosive volcanoes spilled onto the Connecticut Valley floor, which then cooled and hardened into trap rock, and over time, got covered with sediment and stuck into brownstone. After thousands of years of erosion, the weaker brownstone and sedimentary layers were swept away into the Long Island Sound, leaving the more resistant basalt sheets exposed and abruptly tilted. The result is the cliff-faced East Rock we see and enjoy today!

Sources: http://www.wesleyan.edu/ctgeology/Traprock.PDF 

              http://www.mii.org/Minerals/phototraprock.html