I have drawn these cliffs several times and written about them in my new book, "The Brandywine: An Intimate Portrait." At the time that T-Rex walked the earth, these gneiss rocks were already 375 million years old. Unfortunately, most such outcrops were dynamited in the nineteenth century for building farm walls and for paving stones. This one survives in a public park.
The first drawing is Conte crayon on paper toned with ink, based on a pencil sketch. The second was done on-the-spot in Winsor & Newton non-waterproof ink, with its superb range of values from jet black to subtle grays.