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Orthoclase Megacrysts EarthCache

Hidden : 11/30/2016
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Size: Size:   other (other)

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Geocache Description:

Ortho... what?!?

Welcome to Westchester Wilderness Walk, also known as Zofnass Family Preserve. The posted coordinates will bring you to the "Grand Staircase". Use the map linked below, and stay on the trails, there is no need to bushwhack!

In honesty, the Staircase it isn't all that Grand :). It is just a set of stone steps that allow the hiking trail to bypass and climb a 15ft high rock outcrop. But the "staircase" isn't what this Earthcache is about - I just picked this location because its near-vertical rocks make it easy to observe the texture, and because the rock face is bound to remain mostly visible also in deep snow.

Hiking the trails of this preserve, you are bound to notice a peculiar rock type. If you never noticed it before, after this Earthcache, you will :). The rock looks something like this:

Most of the bedrock in Zofnass is ancient Gneiss and Schist, the remnant stubs of prehistoric mountains that have been worn down over the millenia. Both Gneiss and Schist are metamorphic rocks, meaning that they have been folded twisted and compressed after they were deposited. "Bedford Augen Gneiss" is actually a rock type of its own, named after the nearby town, and "Augen" refers to the German word for "Eyes", indicating the oblong-with-tapered-ends pieces of Feldspar present in some of the local bedrock.

In this case, "Augen" doesn't quite fit, I would call it "Bedford Wurm Gneiss" (worm) :). The geo-technical term for this is "porphyroblasts" or "orthoclase megacrysts", which makes you sound smarter than if you just say "large crystals (=megacrysts) of Feldspar (=orthoclase)". What makes things (geologically) a bit more intriguing is that rock crystals form when magma cools, and large crystals are indicative of a very slow cooling process that gives the minerals "time" to align in their natural structure. To have large crystals embedded in a groundmass that has much finer texture means that the magma first cooled slowly, likely deep in the earth, and then cooled rapidly, likely when reaching the surface.But wait! The above describes crystalization in magma, suggesting igneous (volcanic) rock, but what we have here is metamorphic rock (Gneiss). So .. how come? Good question!

First of all, the rocks here were not metamorphed (twisted / compressed) as violently as is the case elsewhere in Westchester. Secondly, if you look closely at the corners of the "wormy" crystals, you'll notice that they are a bit rounded off, suggesting that they were "turned" just a bit during the metamorphosis.

But enough for the theory! Go out there and take a hike, and keep your eyes open on the way to/from GZ, I promise you'll see Gneiss with Orthoclase Megacrysts everywhere. Some of the better chunks are even in the ancient rockwalls that criscross the preserve.

Go to the base (bottom) of the Grand Staircase. There is a "left" and "right" trail up and around the boulder. Look at the vertical rock face right where the two trails split. The "best" specimens are about 5ft up to the right.

(1) Are the large feldspar crystals oriented in every which way, or do they mostly seem to follow a pattern?
(2) Are the large feldspar crystals mostly the same size and shape or how do they differ?
(3) How do the large feldspar crystals at GZ differ from the ones shown on the picture above?

If you like, please post a picture of orthoclase megacrysts that you find anywhere else in the park. Kindly do not post a picture from the ones at GZ, to keep the armchair loggers guessing :)




Trail information and current trail map: Westchester Land Trust
Older trail map: Local Copy

Source: Geology of the Bedford Complex and Surrounding Rocks, Mehdi Alavi, University of Massachusetts Department of Geology, 1975

Thanks to Westchester Land Trust for permission to place this Earthcache!



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