TUSCARORA QUARTZITE

                                       ELBROOK LIMESTONE

  Site Location: 

McCoys Ferry, mileage 110.0/110.29

        Nearest Access: McCoys Ferry, mileage 110.29

  These are two unlikely rock units to describe at the same location, because they differ widely in rock type and also because of their age difference.  But they have been brought together at McCoys by the North Mountain Thrust Fault.  The Tuscarora is a very hard, white, even grained quartzite (sand sized grains of quartz cemented by silica) while the Elbrook is a repetitive assemblage of limestone and dolomite layers.  The Elbrook belongs to that sequence of carbonates that includes the Conococheague and the Rockdale Run that dominates the Shenandoah Valley.  The Tuscarora belongs to a sequence of clastic sediments, notably sandstone and sandy siltstone (shale) that shapes the ridges and valleys from McCoys to Cumberland.  The environment of Elbrook deposition was a marine shelf.  The Tuscarora, however, was a beach deposit where wave action worked over the sand grains and left evidence of that environment in the form of ripple marks and cross beds.  Skolithos worm burrows also occur in the Tuscarora.

  The Tuscarora is a dominating topographic feature because it is mechanically competent and highly resistant to erosion.  At McCoys it can be traced from the stop gate on the Canal, across the recreation area to the river near the boat ramp, crossing the river into West Virginia.  In tracing the Tuscarora, you are also tracing a major fault, the North Mountain Thrust that has transported the carbonate sequence many miles from its eastern basin of deposition.  The Tuscarora, as mechanically strong as it is, exhibits fracturing in the Canal berm exposure above the stop gate.  Once again we find the requirements for the formation of iron ore deposits:  a fault that provides a conduit for iron-rich solutions, an iron source at depth, and limestone in contact with the fault.  Solutions reacted with the limestone precipitating the iron in the form of the mineral limonite.  There was an iron furnace along Green Spring Run that flows into the Potomac at McCoys and iron was recovered for several years from ore banks surrounding North Mountain, locally called Fairview Mountain.  

  Conventional wisdom requires that rock layers that occur above other layers must be younger in age than those underneath them, except when there is thrust faulting.  So here at McCoys the Elbrook, which is about 510 million years old, structurally overlies the Tuscarora  that is about 440 million years old.  Another way to say it is that at McCoys Ferry 70 million years of rock deposits are missing.  Unfortunately, millions of years of erosion by the Potomac River have prevented direct observation of the fault.  The Elbrook, being a soluble carbonate, has been eroded away from the Tuscarora such that the fault contact is not exposed.

  This is the only site along the Canal where the Tuscarora can be observed.  However, at Mile Marker 133 the Tuscarora can be seen across the river where it is the ridge forming cap rock of Cacapon Mountain.  Cacapon Mountain is an anticlinal ridge that plunges downward into the crust near Sir Johns Run such that neither the ridge nor the Tuscarora makes an appearance in Maryland.  This vanishing act allowed the Potomac River to circumnavigate Cacapon Mountain on its way to the sea and escape a confrontation with the Tuscarora.    

Top of page