CONOCOCHEAGUE LIMESTONE

  Site Locations (4): 

Sharpsburg Shelter Caves, mileage 75.70

           Nearest Access: Snyders Landing, mileage 76.65

 Dam 4 Cave, mileage 83.30 and 84.0

           Nearest Access: Dam 4, mileage 84.40

 Two Locks, mileage 107.16

            Nearest Access: Dam 5, mileage 106.80                                                        

Charles Mill, mileage 108.40

          Nearest Access: Four Locks, mileage 108.64

  The Conococheague provides some of the most scenic cliffs along the Canal berm, and dominates the berm more often than other formations.  However, the overall distance of more than 35 miles does not represent a continuous section of Conococheague.  The strong folding and faulting characteristic of the bedrock in the Shenandoan Valley cause repetitions of the rock layers.  If you keep track of the angle of the layers to the horizontal (for example, between Four Locks and Dam 5) you will find repetitions over short distances where the Potomac has cut across folds, the rocks dipping downstream at one point, then upstream further on.  At other sites, the layers stand nearly vertically such as at mileage 107.16 where Canal builders decided to install a dam to pond the Potomac rather than to blast through the rock cliffs.  Of course, dams (seven of them) were required to keep water in the Canal, however, their exact location was often determined by special circumstances such as here.  There is no canal between Two Locks and Dam 5, the dam permitting the barges to be towed in a tranquil river.  Only a path wide enough for the mules was required.

The Conococheague is a very impure limestone that contains layers of silt and dolomite (magnesium rich precipitate).  It is a hard, erosion resistant formation relative to most limestones.  Even though as a carbonate it is soluble, it has been used as a building stone in the Williamsport, MD area, in particular in the construction of the aqueduct over Conococheague Creek, for which it was named.  It also contains a quartz pebble conglomerate layer that can be observed in the middle cliff at mileage 107.16.  This is most unusual.  The Conococheague also contains algal bioherms and some of the dolomite layers exhibit mud cracking.  The layers appear to be cyclical with time, interpreted as episodes of water depth changes.  The environment during deposition was that of a shallow sea on the edge of the North American plate.  At times sediments were deposited at subtidal depths, followed by above tidal episodes and incursions of mud, sand, and even pebbles.  Fossil evidence of the marine environment is the occurrence of Conodonts.  This shallow basin was often indented by barriers to the open sea that created lagoons.  The basin deepened with time losing its episodic character.  To the east, a chain of volcanic islands was beginning to form that provided volcanic ash to the rocks that were deposited after the Conococheague.

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