MATHER GORGE Formation
The metagraywacke in the - Formation
The Early Cambrian (540 million years old) amphibolite or metamorphosed gabbro is emplaced as sills of rich hornblende-rich tabular intrusions, the sills have been placed parallel to the surrounding layers of rock.
The pebbly texture is a result of large crystals of amphibole (of course). The working hypotheses for this location are A) the gabbro was injected as a sill into the metagraywacke/graywacke and then metamorphosed, or B) that the gabbro was already present on the subducting oceanic plate during the Taconian orogeny, and was scraped up as part of the accretionary wedge and metamorphosed with the sediments. [EDIT LATER]
The quartz is located in phyllonite of the Mather Gorge Formation which is greenish-grey, shiny, with fine-grained sheared rock with veins and pods of milky white quartz. Rose Quartz, Milky Quartz and Blue Quartz are all visible in veins, these also contain gold in some locations. An abandoned gold mine located near the current visitor centre on the Maryland side of the park is also visible although it's considered a failed endeavour of the 19th century.
In some locations, the veins are folded back completely on each other, then folded again. Two different deformation events folded the quartz, the first formed isoclinal folds, the second created the much tighter parts of the folds.
These straths are evidence of downcutting by the river in periods of low sea level (ice ages). The very top ledge indicates the position of the oldest riverbed; the next one down represents the lowest level the Potomac incised to during one ice age, and where it stopped for some time. Then another ice age came along, and the river incised downward again to the level it is currently at. This photo shows other evidence of where the river once was: a dissected pothole. The river is actually currently capable of reaching this height during floods, but because potholes take long periods of abrasion to form, we know this one is a feature of the ancient riverbed and not modern flooding (which is relatively short-lived). [EDIT LATER]
National Park Service. (2015, April 10). Geology - Great Falls Park.
Scott Southworth, & Carrie Fingeret. (2000). Geologic Map, Great Falls Park, Virginia. United States Geological Survey (USGS).
Jessica Ball. (2008, April 20). Piedmont geology along the Billy Goat Trail. AUG Blogosphere.