574 THE JOURNAL OF GEOLOGY. 



feet in altitude. Both are separated by steep bare scarps, which 

 are continuous for long distances. Florida, or Boundary Avenue, 

 is at the foot of the younger scarp for several miles, and this 

 scarp is a marked feature all about the Washington amphi- 

 theatre. The earlier Columbia terrace extends widely around 

 Washington, and far up the Potomac valley, at first with rapidly 

 increasing elevation. It is clearly exhibited in the Frederick 

 valley at an altitude of 400 feet, and Mr. Keith has called my 

 attention to extensive terracings in the Goose Creek valley and 

 extending across to the head waters of the Occoquan, which 

 are of earlier Columbia and Inter-Columbia age. In descending 

 the Potomac, the altitudes of the earlier Columbia terrace are 

 found to gradually decrease, and finally it passes beneath the 

 later Columbia terrace and deposits, about thirty miles below 

 Washington. 



In the Baltimore region, the relations are very similar to those 

 in the Potomac valley. The upper part of Baltimore is built 

 mainly on the earlier Columbia terrace at altitudes from 140 to 

 200 feet. The terrace and its deposits have been found to extend 

 up the Jones Falls depression to an altitude of 380 feet in eleven 

 miles, and there are similar relations in the Gunpowder valley,, 

 where the same altitude is finally attained. This is a much 

 steeper slope than exists in the Washington region, and the tilt- 

 ing here attains its maximum degree. Between Washington and 

 Baltimore, there is a moderately wide depression, which holds 

 areas of various sizes of early Columbia terrace at altitudes from 

 180 to 240 feet. This depression expands widely in the region 

 between the Patuxent and the Patapsco, and towards the bay the 

 altitudes gradually decrease to sixty feet near the bay shore. 

 On the eastern shore of Chesapeake Bay, the deposits of the 

 early Columbia terrace are overlain by the later Columbia depos- 

 its at an altitude from five to twenty-five feet above tide water, 

 and to the eastward there is only a very gentle slope seaward. 

 In the region south of the lower Potomac, the relations are simi- 

 lar to those east of the bay. 



The later Columbia terraces extend in a wide belt along: the 



