The Wilmington blue rock, Delaware's most famous rock, underlies both the city of Wilmington and the rolling upland north and east of the city. It is best exposed along the banks of the Brandywine Creek from south of Rockland to the Market Street Bridge. Along this section the Brandywine has carved a deep gorge in the blue rock. The water fall along this four mile gorge is approximately 120', and in the 17th and 18th centuries provided water power for one of the greatest industrial developments in the American colonies. The field trip stops described below are chosen as good examples of blue rock along the Brandywine Creek, and to illustrate how the geology has influenced the development of this area. It is not necessary to visit every stop to become familiar with the blue rocks, you may choose to visit only a few.
The Appalachian Piedmont and Atlantic Coastal Plain are physiographic provinces that are separated by the fall zone. The fall zone (also called the Fall Line) is the contact where the hard crystalline rocks of the Piedmont dip under and disappear beneath the sediments of the Coastal Plain. The landscape and rock types shown in northern Delaware are classical examples of the larger geologic features that dominate the geology of eastern North America.