extra-moraine f^nge in Eastern Pennsylvania. 35 



the eroded country rock. As the ice-extension that produced 

 the erosion antedated the formation of the great moraine, so 

 also the upper clay is of the same age or younger than that 

 formation. We have, therefore, a section that gives the ex- 

 treme limits of the ice age in this region. In studying the 

 till we notice— 



First. The admixture of fresh and oxidized material at all 

 levels, and the uniformity of oxidation at any point along a 

 vertical section. The oxidized portion consists of angular 

 masses of all sizes ; or of angular fragments composing a 

 ground moraine, and of fragments weathered to a spheroidal 

 shape. The fresh portions are sub-angular and glaciated 

 masses ; or rolled bowlders, cobbles, pebbles and gravel. The 

 latter are not found at a uniform level in the deposits ; nor do 

 they occur in a uniform proportion, as sections miles in length 

 have contained but half a dozen of these foreigners and imme- 

 diately adjacent to, and continuous with, this formation the 

 foreigners would form an appreciable proportion of the whole 

 deposit. This curious and persistent admixture proves that 

 oxidation preceded glaciation and, while it may be taken as an 

 evidence of the presence of part of the formation at or near 

 the surface, it can in no sense be used as a criterion of the 

 lapse of time since it was moved to its present position and 

 mixed with the fresh and unoxidized. part. This lapse, on the 

 contrary, is to be estimated by the state of the fresh portion 

 of the admixture. 



Second. The following rule is universal. On going in the 

 track of the glacier southward across the outcrops of slate, 

 limestone and sandstone that make the floor of the great val- 

 ley, we meet the ground moraine composed mainly of com- 

 paratively fresh fragments that, as in the case of the slate, 

 pass gradually into the crushed and bent parts of the outcrop 

 and rest on the solid rock. As we go to the south the slaty 

 fragments become more decomposed and, as we pass from slate 

 to limestone, they are rotted to a clay that retains, however, 

 the cleavages of the small fragments. In the same way the 

 till on the northern part of the limestone belt gradually loses 

 its limestone fragments and retains only the chert at the south. 

 The moraines south of the South Mountain have their southern 

 border formed of weathered gneiss, while the Blue Ridge 

 fresh rocks increase in proportion toward the northern border. 

 This shows that the pre-glacial surface was scraped off by the 

 glacier and exists along the southern part of the formation, 

 where it is mixed with fresher local and foreign material. 



Third. The surface outcrops, .and those capped with clay, 

 of gneiss, sandstone, slate and limestone are almost universally 

 fresh and undecomposed. In the majority of cases there has 



