THE TILL. 291 



mixture of river gravels, as before described. Removing them, it re- 

 sembles ordinary subglacial till, and is found as a mass of angular local 

 fragments or of local paste. The weathering of tills proceeds under the 

 same laws as of ordinar}^ rock of the same nature, onl\^ the broken char- 

 acter of the till invites greater rapidity of decom2:>osition. An old till 

 of limestone or slate will show a clay top that protects to a certain degree 

 the lower parts until the whole mass is resolved into clay. The deposit 

 over this region is not an old till, as there is a constant uniformity of 

 oxidation and decomposition on a vertical section, no matter what the 

 thickness ma}' be. It is fi-equently found jDerfectly fresh, as under the 

 gravel just described, where it is a paste of damourite, holding perfectly 

 fresh and firm fragments of the same rock and river pebbles unoxidized. 

 It rests on the old, rotten, Calciferous sandstone, and the contrast of the 

 light colored till, with its fresh fragments and pebbles, and the soft, yel- 

 loAvish red, underlying rocks (lead colored when fresh) is seen at once, 

 even if there were no sharp line of unconformity between the two. 



Plate 11 shows the lower part of the gravel and the upper 18 inches of 

 till as exposed at the southeastern corner of Eauch's pit. The sequence 

 is, sandy cla}' cap, 1 to 3" inches ; gravel, 2 feet ; till of damourite paste, 

 inclosing fragments of the same with rolled, but not striated pebbles, and 

 having local patches of sandy nature, 2 feet. These rest on decomposed 

 Calciferous sandstone from which the preglacial surface has been re- 

 moved. The thin sandy cap and underlying gravel have lost part of 

 their coloring matter by leaching, with the resultant formation of a 

 conglomerate in the lower inch of gravel, having a limonite matrix, 

 which is seen sharply projecting over the till at the left upper corner of 

 the plate and running diagonally to the right. The ferruginous solution 

 has penetrated the up})er surface of the till to the average depth of one 

 inch and the sandy parts to a greater depth, while the body of till re- 

 mains fresh in color. As the country rock is Calciferous sandstone, the 

 presence of damourite indicates foreign origin of the till, and its fresh- 

 ness under conditions that favor oxidation compels the conclusion that 

 the period of exposure has not been long. 



Tjocally the till is composed princij^ally of gravels,. South of the Blue 

 ridge they are 25 feet thick and freely mixed with huge bowlders, and 

 from Seidcrsville t(^ Temple they form a huge lateral moraine which ex- 

 tends along the northern border of South mountain. One section gives 

 70 feet of tliickness. The average thickness of the till, however, is 3 feet. 

 Immediately south of the Ridge it is not only thick, but rests on the 

 crushed and bent surfaces of the slate, and has but little local rock in 

 its composition. The slate increases in proportion as we go south, and 

 finallv forms the bulk of the mass ; but the gravels and foreign rocks 



