8o FRANCIS P. SHEPARD 



The bowlder bed here consists of a firmly cemented aggregation 

 of limestone, quartzite, and shale blocks with a few fragments of 

 igneous rocks. Limestone is the most abundant constituent, but 

 in some places quartzite bowlders predominate. Many of the 

 pebbles and bowlders are distinctly faceted. Many of them are 

 polished and some rounded as if by stream action. In some cases 

 the quartzite surfaces are so polished as to be dazzling in the sun- 

 light. Markings which suggest glacial striations were found on at 

 least one bowlder. A large block of quartzite was seen to have on 

 its surface a distinct gouge about ten inches long, three inches 

 wide, and half an inch deep. A number of quartzite pebbles had 

 concave surfaces. Evidence from fossils in the bowlders indicates 

 the age of the conglomerate to be younger than Richmond. 



The contact of the bowlder bed with the underlying calcareous 

 shale is distinctly seen. The topmost layer of the shale is contorted 

 and contains pebbles ranging up to six inches in diameter. They 

 undoubtedly came from the same source as the overlying conglom- 

 erate and indicate that the shale was soft when the conglomerate 

 was deposited. 



The bowlder bed is overlaid to the west by massive limestone, 

 with a slight gradation between the two. It is difficult to find the 

 contact because it is covered by debris in most places. 



Before considering the evidence that the formation is tillite, 

 other possible modes of origin may be considered. Is the con- 

 glomerate in question (i) a stream conglomerate, (2) an autoclastic 

 breccia, or (3) an undercHff breccia ? The size and sub-angularity 

 of the bowlders are difficult to explain if the conglomerate is fluvia- 

 tile. Such bowlders are commonly found only in the valleys of 

 mountain streams. As there is no evidence for a change in dip 

 between the underlying and overlying limestone at Sinclair Springs, 

 it is very improbable that there was any intervening uplift which 

 could have led to the development of mountain streams. Further, 

 the gradation between the bowlder bed and the limestone shows 

 that the conglomerate could not have been deposited by mountain 

 torrents. 



Autoclastic breccias are more or less local phenomena. They 

 are generally not confined to one stratigraphic horizon, but are 



