ROCKDALE RUN FORMATION

  Site Location:  McMahon’s Mill, mileage: 88.40

    Nearest Access: McMahon’s Mill, mileage 88.10

  As in the case of the cliffs of the Conococheague persuading Canal builders to locate Dam 5 where they did, a similar situation occurred in this area where cliffs of the Rockdale Run Formation led to the location of Dam 4.  Here was a means of taming the Potomac by creating a deepened slackwater through which canal boats could be towed by mules from the river bank.  This improvement in river navigation mitigated the requirement of blasting a canal prism through solid rock, with black powder.  The Rockdale Run is a succession of limestone and dolomitic layers similar to the Conococheague, but not exhibiting the quartz pebble and siltstone layers found in the latter.  The environment of deposition was that of a shallow inland marine basin that featured episodes in which concentrations of magnesium were introduced with the calcium to form the mineral dolomite; the ratio of magnesium to calcium increasing with time.  While changes in water depth probably did occur, they were more modest than in the case of the Conococheague.  The Rockdale Run, like the Conococheague contains fossils of Conodonts characteristic of marine origins.

  Interesting features in the vicinity of this site are the natural caves that have been carved out of the limestone by water seeping down along solution channels from the surface.  Some of these may have been initiated by the Potomac River when it was flowing at higher elevations than at present creating what is called ‘karst topography’, a surface that is dotted by sink holes and undermined by caverns. 

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