CATOCTIN FORMATION

Site Location: 

 Point of Rocks, mileage 48.40

    Nearest Access: Point of Rocks, mileage 48.40

  The Catoctin is fully observed in the interval between the Point of Rocks railroad tunnel and Mile Marker 50 east of the community of Lander.  The Catoctin is a complex assemblage that includes metamorphosed volcanic/basaltic flows called greenstone, marble (metamorphosed limestone), and metasedimentary units of varying composition.  The Catoctin was invaded by intrusive dikes of rhyolite composition that also became  metamorphosed.  Uranium/lead dating has established a rhyolite age of 571 million years that would then place the age of the bulk of the formation as somewhat older, approaching 600 million years.  Evidence of the buckling caused by plate collisions can be observed at Point of Rocks in the breaks (faults) in the layers exposed in the cliffs.  Catoctin Mountain is an elongated ridge formed by an upbending flexure called an anticline.  As a result, layers of the Catoctin Formation can be seen to slant upwards towards the axis of the ridge from its west and east flanks.  Just to the east of the ridge is the boundary with the much younger rocks of the Triassic Period defined by the Bull Run Mountain Fault, a major fracture that was caused by the separation of the continental plates that stretched the earth’s crust to beyond the breaking point.  The fault permitted iron-rich solutions to move upwards from depth and come into contact with limestone.  The result was large deposits of the iron ore mineral limonite.  These deposits were mined for several years, supporting cold blast iron furnaces at Furnace Mountain in Virginia and the Johnson Furnace near the confluence of Furnace Branch and the Monocacy River.    

  Interpretation of the rock units defines an environment of flows of molten lava similar to those on current day Hawaii.  Between volcanic episodes, lakes formed that became collection basins into which streams deposited sediments and precipitates.  These were the primary rocks that were later altered by heat and pressure to earn the term meta- for metamorphosed.

  Point of Rocks is interesting in that this was the site of a legal battle and at least one Civil War battle.  The C&O Canal and the B&O Railroad began their treks westward on the same day.  At Point of Rocks they had their first confrontation.  There was not room for both to pass between the river and the cliffs of the Catoctin.  The legal battle raged in the courts for years, finally won by the Canal.  So the Canal dug a ditch and the B&O cut a tunnel.  Today the Catoctin cliffs are scarred by drill marks,  room for rail tracks having been blasted from the cliffs.  This occurred after the Canal had ceased operations.  The Canal is clogged with very large boulders, the debris left after the blasting of the cliffs.

  During the Civil War, there was a confrontation of a different type.  The Confederate cavalry of J.S. Mosby (The Gray Ghost) overcame the defending Yankee force, cutting communications to Washington during the attack by Jubal Early.   But that story is too long to fully tell here.

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