Geology of Drip Rock

A few months ago I read a delightful online book and then followed two of the tours verbatim providing a leisurely program of Central Park geology. To make the tour's more convenient, I downloaded & viewed the webpages to my Newton MessagesPad 2100 with Steve Weyer's Newtscape browser and bookmaker/reader. With a small fanny pack Nikon CoolPix 995 and Newton were both tucked away securely as I romped and roamed over the glaciated roches moutonées.

As a result of this I am more acutely aware of the many outcroppings that abound in Central Park. It's like when never noticing the families and children on the Upper West Side until my wife and I started our family. From where did all the strollers come?

Take for instance this photo. Here one can see an erratic, a boulder picked up from the north somewhere and carried down and deposited in Central Park by the Wisconsin ice sheet. Well that's the definition of an erratic but I would be careful to apply it to these found throughout the park. Granted they are large and probably just as heavy but with the blasting out of rocks for nearby roads and walks, I suspect some probability that the landscapers thought it a nice touch.

Drip Rock is a common Manhattan Schist outcropping like the many seen all about the park. It's a pedestal that stands above the moving crowds and horse carriages. Stationary atop of the rock with telescope pointing to celestial south-southeast, the world turns, and daytime astronomical events reveal celestial glories above a horizon of skyscrapers. Now that I've imbibed on some geological awareness, a huge granite dike presents itself at my feet. For the uninformed it could go unnoticed as concrete filling.

This granite dike is an intrusive igneous rock consisting of the minerals feldspar, quartz, and mica. It has a speckled appearance to it. There are two types of granite, granite and granite pegmatite (pegmatite for convenience), and they are distinguished by the size of their grain. The granite grain is smaller and finer, having resulted from rapid cooling. Geologists have determined that the igneous material was injected in to the crevices of a host rock which lost most of its heat.

At the end of the granite dike there are veins of raised, threadlike lines continuing away from the dike, likewise cutting across the grain of the rock. Two can be seen running parallel to the fracture in the rock. Hanley and Graff point out that "

these are veins of quartz thought to have originated when scalding, silica-rich fluids were forced by subterranean pressure into hairline fissures in the rock.

These fissures can be observed throuhout the park on many outcrops, larger and small.
Pegmatite, on the other hand, has larger grains, which the heated host rock provided time for the grains to grow larger. The larger, coarser grains of pegmatite are just as easy to recognize as a foreign object in the schist as the finer granite and can have a pinkish or whitish color.

Manhattan schist exhibits a foliation include that is typical of metamorphic rock. Simply the rock displays a direction or grain. The igneous intrusions can cut across the grain, known as dikes, or run parallel, called sills. Most that I have noticed are dikes like the one illustrated previously.

This large band on the southside of the outcrop to be a granite sill as it is folded into the direction of the rock's grain.


Books of Interest

Hanley, Thomas and M. M. Graff. Rock Trails in Central Park. Greensward Foundation, Inc. 1976.

Raymo, Chet & Maureen Raymo. Written in Stone. Black Dome Press Corp., Hensonville, N.Y. 1989.

Schuberth, Christopher J. The Geology of New York City and environs. Natural History Press, Garden City, N.Y. 1968.


My favorite Central Park map.
- Note to find Drip Rock follow E 62nd St., marked near the lower left corner, across the Park passing Center Drive. Drip Rock is labeled. Across the street to the south is Cop Cot, another area that I use for daytime observation. Nearby my daughter and I catch-n-release fireflies, giving this location a household name Firefly Garden.