William "Sandy" Schenck lead a field trip through the Delaware Piedmont for the A.I. duPont High School Earth Science Class. The trip made use of the Wilmington-Western Railroad and everyone rode the railroad's "Doodle Bug." Activities included up close examinations of rock and mineral features and even "Panning for Garnets" at Brandywine Springs Park.
Wissahickon gneisses and amphibolites are exposed in the railroad cut near Yorklyn. Here the rocks are unusual because the layering is accentuated by the presence of fault gouge between the layers. Fault gouge forms as movement along a fault in hard, brittle rocks crushes and grinds the rocks into a powder. Gouge was a term used by miners because they could easily "gouge" it out of the rock. Here the gouge "weathered out" leaving deep indentations that emphasize the layering and the tilt, which is to the southeast at an angle of about 45 degrees.
This report accompanies a new map that revises the original bedrock geologic maps of the Delaware Piedmont compiled by Woodruff and Thompson and published by the Delaware Geological Survey (DGS) in 1972 and 1975. Combined detailed mapping, petrography, geochemistry, and U-Pb geochronology have allowed us to redefine two rock units and formally recognize eleven new units. A section of the Pennsylvania Piedmont is included on the new map to show the entire extent of the Mill Creek Nappe and the Arden Plutonic Supersuite.
The effect of rapid growth in the Hockessin and Pleasant Hill areas in northern Delaware has caused concern about possible declines in ground-water recharge to the underlying Cockeysville Formation. The Cockeysville is a major source of ground water (aquifer) in the Hockessin area from which about 1.5 million gallons of water per day is withdrawn for public water supply, even though it receives recharge over a relatively small area of 1.6 square miles. The Cockeysville in the Pleasant Hill area is currently used as a source at water supply for individual domestic users and one school.
GeoAdventures are designed to allow the reader to learn about a particular geologic point of interest in Delaware’s Piedmont province and then take a short field trip to that area. Want to know more about the Wilmington blue rock or Brandywine blue granite? Take the Wilmington Blue Rock GeoAdventure and go see just what the blue rock looks like.