THE DWYKA FORMATION 405 



expression. The tillite contains abundant fragments, small and large, of 

 many kinds of rocks. It was easy to find striated stones on the slope of 

 the ridge, where they had lately been weathered from the matrix. It was 

 noteworthy that this heavy tillite sheet lay evenly on the basal stratified 

 member without any discoverable disturbance or unconformity. In this 

 it corresponds to various sheets of till in the upper Mississippi valley, 

 which frequently lie conformably on stratified deposits. It is evident that 

 the tillite indicates the arrival of the ice-sheet, whose coming had been 

 foreshadowed by stones in the basal member, and that the ice-sheet acted 

 here as an aggrading agent, in striking contrast to its behavior farther 

 north, as will appear later. 



The third member of the Dwyka is worn down in a well defined longi- 

 tudinal valley, although it is not distinguishable in appearance from the 

 tillite of the ridges that inclose it. Its structure is well shown in abun- 

 dant exposures in the dry channel of Wittebergs river, which turns along 

 the valley for half a mile between two notches. Small boulders of many 

 kinds are here seen in the tillite. This valley is shown in plate 52, figure 

 1, between ridges of the second and fourth members, which turn gently 

 to the left in the distance. 



The fourth member makes a ridge like the second. It is a sheet of 

 tillite, but seems to be divided about the middle by a weaker sheet — per- 

 haps by some stratified layers that we did not see — for the ridge is some- 

 what grooved along the top on each side of the notch that we came 

 through. 



The fifth member is again stratified, and here stands vertical, as in 

 plate 51, figure 1. It contains scattered stones, like the basal member. 

 The upper surface of the fourth member and the lower surface of the 

 sixth seemed to be closely conformable to the stratification of the fifth. 

 The fifth layer was taken to indicate a withdrawal of the ice-sheet which 

 had formed the second, third, and fourth members and a resumption of 

 aqueous conditions similar to those which prevailed during the deposi- 

 tion of the basal member. 



The sixth member is a resistant, ridge-making sheet of tillite, not so 

 thick as the fourth, but of the same nature. It appeared to be conform- 

 ably followed by the overlying sandstones and shales. Like the other 

 unstratified sheets, this one weathered on transverse joints and surfaces of 

 incipient schistosity into pillows and spikes, giving the crest and upper 

 slopes of the ridge the appearance of being built up in part of bales and 

 sticks laid crosswise. This sheet of tillite manifestly indicates a return 

 of the aggrading ice-sheet — that is, of an ice-sheet which by its own 

 weakening action here laid down the detritus that it had dragged from 

 elsewhere. 



