Chadwick Preserve POI C7 Picnic Rock
Introduction
Text-to-speech Audio
Proceed about 10 yards down the long Point Trail from C6 Giant Sycamore and then 10 yards to the right by Ridley Creek. The wide rock jutting into the creek is Picnic Rock. It is composed of schist, another metamorphic rock common to the crystalline Piedmont of southeastern Pennsylvania.
Images
Picnic Rock from the Creekside
Close up of Picnic Rock
Backstory and Context
Text-to-speech Audio
This giant boulder of Wissahickon schist most likely detached many thousands of years ago from the steep hill on the opposite side of Ridley Creek and rolled down to its current dramatic location on the creek bed. The Wissahickon Formation is a mapped bedrock unit in Pennsylvania, New Jersey, and Delaware. It is named for the Wissahickon gorge in Fairmount Park, Philadelphia. The Wissahickon is described as a pelitic schist and gneiss with interlayers of quartzite.
Color is highly variable which is its appeal for construction. A general description is a silver to brown garnet mica-schist. The highly variable nature of this rock type is also why the metamorphic grade (amount of heat and pressure involved in its metamorphism) is also complex. The existence of the minerals biotite, garnet, staurolite, and kyanite all imply a low-intermediate to high metamorphic grade, depending on location.
Scouring by floodwaters carrying sand and gravel has smoothed its surface. Find a workable path down the eroding silted edge to access Picnic Rock. Visitors are welcome to sit out in the sun and enjoy the soothing sound of the creek’s rapids, the peacefulness of this isolated valley, and the comings and goings of the local wildlife. If you’re lucky you may spot a bald eagle fishing in the creek. It’s a great spot for a picnic, but don’t forget to take your trash with you and please respect the water and all that lives in it.
Sources
Latham, Dr. Roger. Picnic Rock. Notes for the Chadwick Preserve. Published August 16th, 2023.
Blackmer, G.C., (2005). Preliminary Bedrock Geologic Map of a Portion of the Wilmington 30- by 60-Minute Quadrangle, Southeastern Pennsylvania. Pennsylvania Geologic Survey, Open-File Report OFBM-05-01.0.