Cacti on the Billy Goat Trail
Yesterday, I lead a NOVA field studies trip to the Billy Goat Trail, and we were pleased to see that the recent rains had plumped up the big patch of Opuntia (beavertail) cactus near the boulder of Seneca Sandstone. These are native cacti which grow on poor soils in the area. These soils are poor because periodically the Bear Island strath (bedrock terrace), where the Billy Goat Trail is located, is scoured by the Potomac River's floods. These cacti are growing essentially in pine and juniper needles on top of bare rock (metagraywacke). What a beautiful sight!
5 Comments:
In general are most of the gray rocks of the Appalachian mountains - Gray metawacke?
I am thinking of all the rocks near Thurmont like wolf's rock and Catoctin falls - and the rocks out at Sideling hill. I'll have to go check them out again. Even the rocks you shown in DC with the agates in them - were they Gray Metawacke?
Hey, I was happy to put the bumper sticker prize you sent for answering the rock sequence question on my car! It looks great there!
Great picture of cacti in the area. I never would have guessed we had native cacti!
Thomas Donlon
So neat to have prickly pear there. Don't remember ever seeing any back in the 60's and 70's when I was just a kid. Or maybe I've just forgotten.
Hey there Thomas --
It's Metagraywacke, not Gray Metawacke. Graywacke is a kind of sandstone -- sand mixed with dark mud, that is typical of deposition in deep ocean basins.
No, not all gray rocks in the Appalachians are graywacke (or metagraywacke). But there are examples of each -- in the Piedmont province, there are metagraywackes in the Mather Gorge Formation, and probably the Sykesville Formation and Laurel Formation. (I'll post in the future about a potential re-interpretation of these rocks.)
Anyhow, there are also un-metamorphosed graywackes in the Valley and Ridge province -- the Martinsburg Formation is a prime example.
I don't know what you mean about agates -- what are you referring to there? (Please specify time, location, context -- there are no bona fide agates in DC rocks. (I saw you mentioned that in a previous post, but didn't have time/inclination to correct it... Now that you've mentioned it again, I feel obligated to correct the misconception.)
C
Callan, probably the correct word for what I was thinking is garnets rather than agates? We saw them on the walking/bicycle trail after leaving the DC zoo. They were evidence that the rocks were once about 15 kilometers under the surface.
Anyway, it is great that you share your knowledge with us. I've got so much to learn!
Thomas Donlon
Aha -- yes. Garnets, not agates.
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