POOLESVILLE MEMBER, MANASSAS SANDSTONE

     Site Locations:   NPS Map: both locales    

Seneca Quarries, mileage: 23.13     NPS Map    USGS Map                   

        Nearest Access: Riley's Lock, mileage 22.82                                                              

Marble Quarry Overnight Camp, mileage 38.50     NPS Map    USGS Map

        Nearest Access: Dickerson Regional Park, mileage 39.63

  This sandstone is locally referred to as the Seneca Red Sandstone.  It was a favorite building stone being transported to Washington and to several building sites along the Canal.  Dressed stone blocks are found as far west as Williamsport (Conococheague Aqueduct), as far east as Seven Locks, and across the river at Great Falls Park Virginia (Patomack Canal lift lock).  At Seneca (Riley's Lock), the ruined walls of a stone cutting mill can be seen off the berm of the canal.  The series of quarries that fed the mill follows the berm upstream from Seneca Creek for several hundred yards.  The lock keepers house and the Seneca Creek Aqueduct are constructed of Poolesville as is the Smithsonian Castle on the Mall in Washington.                                                                   

  This is a gray, pinkish gray, or reddish brown sandstone that is fine to coarse grained, arkosic and containing minute flakes of mica.  It may locally contain small pebbles.  It is quarried where it is thick bedded.  This sandstone is easily identified in locks and Canal buildings where exhibits the reddish brown coloration.  It was deposited during a rifting of the continental plates that created a large depositional basin with its western margin the highlands along the Catoctin Mountain and the Bull Run Mountain.  This margin is the Bull Run Mountain Fault along which the crust broke and slipped down to the east to permit a depression that then received rock debris from the rock formations to the west.  Some of the debris was water worn and was deposited as well sorted and bedded sediments.  The rifting and subsequent deposition of the Poolesville occurred some 200 million years ago.  Compare this age with the ages of the rocks adjacent to it up and down the river.  Approximately 360 million years of rock record has been removed.  The Poolesville was deposited during upper Triassic time, bordering closely in time with the age of the dinosaurs; in fact, tracks have been found in specimens of these rocks to the south.

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