SILURIAN TILLITE IN BRITISH COLUMBIA 8 1 



likely to cross bedding planes, while the formation in question con- 

 stitutes a distinct horizon traceable for a number of miles. The 

 diversity of types of rock among the bowlders in the Windermere 

 Creek occurrence is not characteristic of an autoclastic breccia, and 

 the thickness of the formation is also against this interpretation. 



UndercHff breccias have angular unsorted material such as 

 found in this formation, but the possibiHty of adjacent chffs at this 

 time is slight, because of the gradation into overlying and under- 

 lying limestone, which shows only shght interruption of marine 

 conditions. The great continuation of the formation both along the 

 range and in a vertical direction (it has a known vertical range of 

 more than 3,000 feet) are altogether against a cliff-base origin. 



Qn the other hand, evidence for glacial action is strong. It 

 includes (i) the finding of probable striations, (2) the sub-angular- 

 ity of the bowlders, (3) the distinct faceting of some of the bowlders, 

 (4) the heterogeneous mixture of the bowlders without assortment 

 or stratification, (5) the occurrence of grooving and concave faces 

 on pebbles, and (6) the occurrence of Silurian tillite in southeastern 

 Alaska,^ 900 miles away, also is significant. 



Thus it seems possible that glacier ice existed at this period in 

 British Columbia and southeastern Alaska. Since there are marine 

 deposits on both sides of the formations in Alaska as well as in 

 British Columbia, it is possible that the glaciers came out into 

 shallow seas in these places. The absence of weathering at the top 

 of the conglomerate points to encroachment of the sea before such 

 weathering took place. The gradation into limestone in several 

 instances in British Columbia suggests that icebergs coming from 

 the retreating or advancing ice fronts distributed occasional bowl- 

 ders among the limestone deposits. It is not surprising that this 

 formation should continue over such great distance without a 

 break, when it is considered that sea deposits covered it before it 

 could be attacked by erosion. 



^ E. Kirk, Amer. Jour. Sci., Fourth Series, September, 1918, p. 511. 



