METAGRANITE

Site Locations:  None on the Canal

  There are no observable exposures of the metagranite along the towpath.  However, the geologic history of the rocks of the Potomac River Valley would be incomplete without a brief description of the rock that exists between the communities of Lander and Weverton.  Metagranite is included here because it is the oldest rock along the Canal having been dated at 1,111 million years (that is, one billion plus).  Metagranite is the basement rock upon which all other rock formations have been deposited and is the surface rock of an approximate seven-mile wide area that extends for many miles southwest across Virginia and northeast across Maryland.  Please note that more than 500 million years separate the deposition of the metagranite and that of the Catoctin Formation, a long interval during which there is no rock record of geologic processes.  Since the later rock formations exhibit an unconformable relation to the basement, the interval is interpreted as being one of erosion without evidence of deposition within the local geographic area.

  Metagranite began its existence as a molten body that was forced up through previously existing rock where it cooled and crystallized as an assemblage of minerals that collectively define ‘granite’.  It differs from the Catoctin both in composition and the fact that it formed within the crust rather than spilling out onto the surface.  Heat and pressure later altered the granite to a metamorphosed assemblage of minerals differing from the original minerals in their stability to the elevated conditions.  Plate collisions distorted it and later molten bodies intruded it.

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