EARLY GLACIAL PLAINS TRENCHED BY INTERGLACIAL VALLEYS. 473 



ference that the trench was cut before the moraines were pushed across it, 

 and before the moraine-derived gravels could be carried down into it. The 

 trench therefore represents the interval between the earlier and the later 

 glacial epochs. I have placed in manuscript elsewhere the fuller facts 

 upon which these brief statements rest, and they will appear in print in 

 time. 



The Evidence drawn from the Susquehanna. — If we pass over the Susque- 

 hanna valley we find like phenomena. These have been brought out by 

 Mr. McGee and others, and I need only refer to them because of their con- 

 nection with that which I have already presented. Here we find old benches 

 covered with rounded pebbles — some of which are glaciated — reaching to a 

 similar height of about 250 feet above the present Susquehanna river. 

 There are glaciated pebbles at higher altitudes, but I have taken the more 

 moderate figure because it is a safe one. Near Sunbury glaciated stones 

 were found by Professor Salisbury about six hundred feet above the present 

 river. Below these high terraces, and in the valley excavated out of the 

 plain from which they were derived, we find a lower terrace sixty or seventy 

 feet in height, of newer and distinctive aspect. Above Berwick this lower 

 terrace connects itself definitely with the terminal moraine, which there 

 crosses the river. The terrace rises rapidly as it joins this moraine, as is 

 the habit of moraine-headed terraces, and reaches an altitude of 100 to 150 

 feet as it merges into the moraine. But it is still much below the old ter- 

 races, from which it is sharply distinguished by its freshness and other 

 marks of youth and by its constituent material. 



It appears therefore that at this point a deep trench was cut in the flood- 

 plain of which the old terraces are the remnants before the formation of the 

 later moraine and of the valley deposits that sprang from it. 



The Evidence drawn from the Delaware. — If we cross the Appalachian 

 crest to the Delaware valley we find analogous facts, which are more famil- 

 iar through the writings of several geologists. Many years ago Professor 

 Lewis called attention to the earlier and later deposits of that region, though 

 he did not give them the interpretation I shall place upon them here, which 

 coincides essentially with that of McGee. As we follow up the valley toward 

 Belvidere, where the moraine crosses the Delaware, we find old terraces 

 reaching up to about 240 by 250 feet, upon which are rounded pebbles and 

 glaciated stones, indicating an origin in the earlier stage of glaciation. Cut- 

 ting through these old plains and the rock below we find the deep trench 

 in which the later deposits have been placed. These later gravel deposits 

 originating with the moraine at a height of somewhat above 150 feet, rap- 

 idly decline to about 85 feet a few miles above Lewisburg, opposite a point 

 where the older terrace rises to about 250. The measure of the interval 

 here is some 250 to 300 feet of rock-cutting. 



