The Brandywine Blue Gneiss is also known as the Wilmington Blue Rocks.  The rocks were formed during the Taconic orgeny or mountain building phase when the proto North American continent collided with an island arc off the coast.  The collision formed a large mountain range that preceded the Appalachians.  The Brandywinde Blue Gneiss is metamorphosed volcanic rocks caught up in the pressures and temperatures of the collision zone.