534 PRINCIPLES OF STRATIGRAPHY 



mass are apt to be incorporated in the forming deposit, but such boul- 

 ders are by virtue of their composition subject to speedy destruc- 

 tion. As a result a depression will be formed in the glacial deposit, 

 due to the caving-in of the overlying material into the hollow left 

 by the vanishing ice boulder. Such kettle holes are common in 

 both modified and unmodified glacial deposits and serve to identify 

 them as such. 



Ancient Glacial Deposits. 



Boulder clays and modified glacial deposits are found at a num- 

 ber of geological horizons from the pre-Cambric onward. The 

 rock formed by the consolidation of such deposits is known as 

 tillite and its essential character is found in the heterogeneous mix- 

 ture of the material, the absence of stratification and the striation 

 of the larger blocks, and the fragments of dense texture. Several 

 of the more prominent cases may be discussed in some detail. 



The Prc-Cainbric Tillite of Canada. In the Lower LIuronian 

 of the Cobalt region "in Canada an extensive boulder bed and tillite 

 have been found, indicating glacial origin. (Coleman-2, 3, 4, 5.) 

 The tillite is a consolidated boulder clay or till and the boulders 

 are angular or subangular. and not infrequently striated, but no 

 striation of the underlying rock has yet been observed. The boul- 

 ders are often 2 or 3 feet or more in diameter and distant a couple 

 of miles from the exposures of the rocks. Some of the boulders of 

 this conglomerate are tons in weight, and the areal extent of the 

 mass has been traced over i.ooo miles east and west and 750 miles 

 north and south, while the thickness at Cobalt is about five hundred 

 feet. The boulder clay of this horizon and that of the Permo-Car- 

 bonic and other regions is almost indistinguishable in hand speci- 

 mens or in thin sections under the microscope. Some stratified 

 slaty beds have been found at Cobalt in the great mass of unstrati- 

 fied conglomerate suggesting a possible interglacial period. The 

 glacial origin of these has been questioned by Knight (17) and 

 others. 



Cambric Glacial Deposits. These have been recorded from 

 northern Norway, where in the Varanger fjord an unstratified tillite 

 with striated boulders rests upon a striated rock surface of the pre- 

 Cambric Gaisa series and is succeeded by beds of Cambric age. 

 (Strahan-29. ) From the Yantzi Canyon in China, lat. 31°, Bailey 

 Willis (33) has obtained striated boulders occurring in tillite per- 

 haps 150 feet thick and overlain by Middle Cambric marine deposits. 

 Still more striking and better known are the Cambric tillites of South 



