TOMSTOWN FORMATION

  Site Locations: 

Huckleberry Hill Overnight Camp, mileage 62.90

          Nearest Access: Dargan Bend Recreation Area, mileage 64.89

Dargan Bend, mileage 65.30

          Nearest Access: Dargan Bend Recreation Area, mileage 64.89

Giant Steps, mileage 68.0

     Nearest Access: Mt. Lock Recreation Area, mileage 67.16

  The Tomstown is the first of a massive sequence of carbonate rocks that in aggregate are about 10,000 feet thick, and took an estimated 40 million years to accumulate.  They extend from Dargan to McCoys Ferry at Mile Marker 110 (a distance along the Canal of 45 miles), and are the predominant bedrock of the Shenandoah Valley.  The Tomstown differs from the younger succession of carbonates in that it is primarily a dolomite in which magnesium is a primary constituent of the rock.  The Tomstown is a calcium magnesium carbonate whereas most limestones are calcium carbonate. 

  The Tomstown provides evidence that a passive continental margin was established following the transgressive shorelines indicated by beach and mud flat environments of the Weverton/Antietam sequence.  During Tomstown time, the decidedly open ocean environment went from shallow shelf to deep shelf, the middle member having been deposited in the deepest water.  The Tomstown contains a cone-shaped fossil, Salterella, that lived in the marine environment. 

  The Dargan site is especially interesting in the remains of buildings that attest to manganese recovery, manganese nodules having been discovered following excavations to build the Canal.  When manganese could no longer be recovered, the furnaces became kilns for limestone.  The observed tunnel was dug to allow the Tomstown to be quarried upstream of the kilns and transported along the Canal berm to the kilns.  The lime that was produced had a very high magnesium content that meant that the product was mainly used for agricultural purposes rather than for cement.  Yellow colored deposits that contain pebbles of differing sizes and compositions can be observed just upstream of the tunnel along the berm.  These are the remains of a sinkhole that was formed in the Tomstown and became filled with debris deposited by the Potomac River.  This is evidence that the Potomac once flowed at a higher elevation, at the level of the bank above the sinkhole.  You are observing the sinkhole in cross section along with solution caves further upstream in the quarry walls.  Finally, large specimens of limonite (iron ore) have been found here.  Prospect holes for iron dug in the Dargan area proved the existence of iron ore at some depth, however, there is no record of iron recovery here.  That same fault, Keedysville Detachment, that was instrumental in making Huckleberry Hill a very rich iron ore bank was active here, but was not exploited for iron.  Beautifully formed specimens of the kidney-shaped ore mineral of manganese, pyrolusite, are still discovered in this area.   

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