Tripping over shiny rocks while hiking: Delaware’s Piedmont

Saturday was the kind of gorgeous day that makes it impossible to stay inside, so I set out to explore White Clay Creek State Park. The sun was shining, hawks were soaring on the gusts…

…and the mud was glittery. Seriously, it looked like the dirty aftermath of a craft-store glitter aisle explosion. Further up the slope the trail was fortified with chunks of feldspar as well as the mica-rich rock that crumbled into small sparkling flakes in my hand. It took me aback; the last time I’d seen that kind of soil was in outcrops of the Precambrian Pony formation in the Tobacco Root Mountains of Montana. Somehow I thought of Delaware as a flat, sediment-covered landscape without exposed bedrock, but that just shows how little I know about Delaware.

Later on at the Bryan’s Field trail head, a sign informed me that I was standing on the 6% of Delaware that’s in the Piedmont region of the Appalachians. Suddenly the glittering mud made a lot more sense.

My purse is now mostly full of rocks gleaned from a gravel bar in the stream– amphibolite, smoky quartz, translucent rose-colored feldspar, and bits of mica-rich gneiss.

So, what’s a geologist to do but find a map? Luckily the Delaware Geological Survey has a great geologic map of Newcastle County.

According to that map I had been hiking on the Wissahickon Formation (COwf, in geologists’ shorthand), whose rocks were laid down between the Cambrian and Ordovician periods.

Its official description is:

Interlayered psammitic and pelitic gneiss with amphibolite. Psammitic gneiss is a medium- to fine-grained biotite-plagioclase-quartz gneiss with or without small garnets. Contacts with pelitic gneiss are gradational. Pelitic gneiss is medium- to coarse-grained garnet-sillimanite-biotite-plagioclase-quartz gneiss. Unit has a streaked or flasered appearance owing to the segregation of garnet-sillimanite-biotite stringers that surround lenses of quartz and feldspar. Throughout, layers of fine to medium-grained amphibolite composed of plagioclase and hornblende, several inches to <30 feet thick or as large massive bodies, are in sharp contact with the psammitic and pelitic gneisses. Granitic pegmatite is ubiquitous and occurs at all scales.

And to translate that out of geologese:

Flasered = A dynamically metamorphosed rock in which lenses or layers of original or relatively unaltered granular materials are surrounded by a matrix of highly sheared and crushed material, giving the appearance of a jumbled flow structure

Gradational contact= as opposed to a contact that shows a sharp contrast between two types of rocks, this contact shows a more gradual shift. In gneiss, which is layered by definition, a gradational contact can show up slowly changing composition of those layers on the scale of a few inches to many feet.

And the MS Paint breakdown of types of gneiss:

Psammitic Gneiss

Pelitic GneissFor a clear geologic history of the Piedmont region of Delaware, you can check out the Delaware Geological Survey’s explanation. The gist of the story is that these gneisses are the metamorphosed form of sediments deposited in a deep ocean basin between a volcanic island archipelago (whose remains form the nearby amphibolites and Wilmington “blue rocks”) and the continental shelf. These features were scraped onto the North American plate during the Taconic mountain-building event around 440 million years ago, in the late Ordovician.

And as a bonus, while researching for this post I stumbled across the answer to my question about the beautiful green stone I’ve seen on the facades of buildings around here: serpentinite! It underlies the pine barrens, which are “barren” because the high chromium content of the rock that makes it that fetching shade of green also poisons most plant life. Serpentinite is formed by the hydration and metamorphosis of some ultramafic (low in silica and potassium, high in iron and magnesium) igneous rocks of the ocean crust, which are represented by the black blobs in the above diagram.

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