496 A. Lindenkohl — Post-glacial Subsidence 



Ohio Railroad bridge above Havre de Grace, kindly furnished 

 by Mr. Chas. F. Mayer, the President of ' the road, shows a con- 

 siderable layer of mud intervening between the bottom of the 

 river and the rocky granite floor. This layer has a thickness 

 of fifty-nine feet in the west, and over seventy feet in the east 

 channel. The river would most certainly not have cut a chan- 

 nel into one of the hardest of rocks if there had not existed, 

 at some time, a physical necessity for it, and the amount of 

 filling or ik packing " of mud enables us to estimate the depth of 

 the river at that time. Assuming the discharge to be stationary, 

 we find that, supposing the mud to be removed, the river 

 could be lowered forty-three feet and yet find sufficient space 

 for the passage of its waters. The next profile (p. 194) is from 

 the crossing of the Pennsylvania Railroad at Havre de Grace, 

 about one mile to the southward of the B. cfe O. R. R. bridge. 

 This profile was obtained from Mr. G-. B. Roberts, President 

 of the Penna. R. R. It shows the greatest depth of mud, 113 

 feet under the wharf at Perryville. It would appear then that 

 the channel of the river ran very closely to its eastern shore 

 which was then several hundred feet farther inland. The rock 

 is stated by Mr. McGee to dip under the level of the river about 

 one-quarter of a mile from the railroad bridge. A similar cal- 

 culation for the level of the river with the rocky floor for its 

 bed. instead of the muddy bottom, gives fifty feet below the 

 present surface. These two estimates taken in connection with 

 the result of borings at Fishing Battery mentioned above, would 

 appear to prove that at the time when the level of the Chesa- 

 peake was forty-eight or fifty feet lower with respect to the 

 land than at present. Perryville and not Port Deposit, was at 

 the head of tide and that strong currents swept down the Sus- 

 quehanna past and on both sides of Watson Island, plowing 

 into the clays of the coastal plain to a depth of ninety feet or 

 more. 



The Potomac being a tributary of Chesapeake Bay, we 

 should naturally expect indications of a sinking of the land at 

 the head of tide, similar to those of the Susquehanna. An 

 examination of several profiles of the river at the Free 

 Bridge in Georgetown (the former Aqueduct Bridge, built 

 about 1840) shows the excavation of the channel to be of 

 quite different shape from that of the Susquehanna ; it is 

 fiat at the bottom and only reaches to the depth of thirty-five 

 feet from the surface. There was considerable "packing" 

 by mud before the bridge was built, about thirteen feet 

 thickness on an average. The cross-section of the river was 

 considerably curtailed by the construction of the bridge. 

 The river has tried to regain its former status and nearly 

 succeeded in this effort, by removing the greater part of the 



