The stretch of Forbidden Drive that runs from Bell's Mill Road to Northwestern Avenue preserves an ancient plate boundary that separates rocks that have been part of North America for more than 1 billion years from oceanic crust and mantle that were added to the continent 400 to 450 million years ago.  It is an exciting place for a geologist because it allows us to gain insight into the workings of our planet that build and destroy mountains.

    The topographic map on this page shows where each outcrop discussed can be found.  To learn more about the rocks click on the links labeled stop 1 or stop 2, etc.  Each link includes photographs and a brief written description of the rocks and their significance to the geologic history of the region.

STOP 1 -- WissahickonFormation with schist and quartzite.

STOP 2 -- Serpentinite, a phyllite with chemical characteristics of oceanic mantle rocks.

STOP 3 -- Wissahickon Formation

STOP 4 -- A granodioritic dike intruding the Wissahickon Formation is exposed in a small quarry.

STOP 5 -- Baltimore Gneiss exposed on the north side of the Rosemont Fault that separates ancient North America from accreted terrain.

Simplified geologic map of the area.