GEOLOGIC COLUMN---POTOMAC RIVER VALLEY

This is a chronological listing of the rock formations that occur along the Potomac River. Most of them are exposed along the C&O Canal and are described in this guide in the geologic attractions.  The list is included to clarify the sequence of deposition of the rock types: sandstone/quartzite, limestone/dolomite, siltstone/shale, conglomerate, igneous, metamorphics, unconsolidated.

 NOTES:  The above sequence represents essentially a continuum in deposition from the Swift Run through the Purslane.  The older formations listed here were disconnected in time and space from the continuum.  The metagranite is the oldest rock following which there was a very long period devoid of identifiable deposition.  All of the Proterozoic formations were highly metamorphosed and deformed; some were additionally intruded by rising molten bodies.  The oldest of the Cambrian formations also were altered by heat and pressure, but not to the same degree as the Proterozoic rocks.  The Triassic formations also form a discontinuous group having been deposited in a tectonically formed basin on rocks that are very much older.  The depositional history starting with the Weverton can be generalized.  During the sequence of Weverton to Tomstown the rocks are clastics, primarily beach sands and silts punctuated by conglomerates.  Then there is a very long episode of carbonate precipitation, Tomstown to Martinsburg, during which there were times when the abundance of magnesium caused dolomite to form.  The Martinsburg marks a return to clastic deposition along shorelines where beach sands predominated.  Some of the sediments were deposited in decidedly oxidizing environments that caused red beds to form.  The Wills Creek marks a transition back to a carbonate sequence in which the Tonoloway excels in purity.  Finally the Oriskany ushers in a return to clastic deposition in beach and muddy marine environments culminating in the Pennsylvanian coal forming swamps.                            

(The years listed for each period is the end of each period.  For example, 505 marks the end of the Cambrian, therefore Ordovician time is from 505 to 438 million years ago.)

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