Figure 11. Photographs of Stop 4, Great Falls Park, Va. A, Looking north at steeply dipping
layers of folded metagraywacke and schist of the Mather Gorge Formation
in the Potomac River at the type-locality. B, Closeup of a boulder of metagraywacke
showing graded beds and subangular feldspar deposited in turbidite sequences.
Pen is 14 cm long. C, Looking east at the lamprophyre
dikes (see arrows) that intruded north-dipping fractures in metagraywacke
and schist of the Mather Gorge Formation. The dikes have a ~360-Ma conventional
K-Ar biotite cooling age. Potomac River is in the foreground.