LEESBURG MEMBER

 BALLS BLUFF SILTSTONE

Site Locations:

  Marble Quarry Overnight Camp, mileage 38.35

              Nearest Access: Dickerson Regional Park, mileage 39.63

    Rock Hall, mileage 47.10

             Nearest Access: Point of Rocks, mileage 48.40

  This unique rock has been locally called ‘calico marble’ or ‘Potomac marble’.  However, it is actually a limestone conglomerate since it is composed of limestone and other rock fragments from previously existing rock formations cemented together by calcium carbonate, the major component of all limestone.  It has not experienced the heat and pressure that would have caused metamorphism to marble.  The Leesburg was formed about 210 million years ago in the same Triassic-age basin as the red Poolesville sandstone.  The Leesburg generally has a reddish coloration due to the iron content in the silt that was incorporated into the cement.  The local names for the Leesburg originate in the fact that it was highly prized as a decorative stone having been quarried on both sides of the Potomac, dressed, and polished to produce the columns in the U.S. Capitol Statuary Hall.  Because it is composed to a high degree of limestone fragments from the Frederick Limestone (supporting a cement industry northeast of Point of Rocks), it was also quarried and fired in kilns to produce agricultural lime.  Kilns of this sort can be seen at Dargan Bend (see Tomstown Formation) and at Round Top (see Tonoloway Limestone).                       

  The Leesburg is a smorgasbord providing specimens of rock formations that predated it along and to the west of the Bull Run Mountain Fault.  That fault defines the eastern base of Catoctin Mountain in Virginia and Maryland.  You will note that the fragments are sub-angular in shape and vary widely in size.  These fragments accumulated in coalescing debris flows off highland cliffs to the west that were formed by the tectonic activity.  There was some localized transport by water but very little in the way of the rounding that is characteristic of pebbles found today in the Potomac River.  The deposition of the calcium carbonate cement occurred when the basin was inundated with water into which  limestone, probably from the Frederick, had dissolved.

Top of Page