Ere we reached the Great Falls the rocks lay bared for a great distance, as ruggedly as if they were upon their edges, somewhat resembling the effect that would be produced by the instantaneous congelation of a sea highly cuspated. As my friends very kindly permitted me to leave the boat whenever I desired to examine any locality, I descended to these rocks, and found reason to believe that they had been brought to that state by the action of the water: remains of pot-holes were numerous, some of which had been two and three feet in diameter. Here, undoubtedly, had been a ledge across the river, which at this place is 168 feet above the level of tide water, giving about 14 feet slope per mile for the distance from George Town. Like most other streams, the Potomac has worked its channels through the rocks by retrocession, and in long periods of time has worn down this ledge by first forming pot-holes, and then attenuating and breaking them up. In this particular place the very rugged appearance has been materially caused by the inclination of the strata, formed of a coarse micaceous slate.
Reading Water: Lessons From The River (Capital Dis depuis giant message
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