58 SEASONAL DEPOSITION IN AQUEO-GLACIAL SEDIMENTS. 
“Where the stratified beds of the Dwyka are devoid of pebbles, they are in places 
exactly similar to and really not to be distinguished from the Ecca shales, overlying the Dwyka 
Conglomerate. The Dwyka Conglomerate passes gradually into the Ecca shales and there 
is no real difference whatever between the matrix of the Dwyka Conglomerate and the 
material of the Ecca shales. The Ecca beds are entirely devoid of pebbles, with the excep- 
tion of some rare big boulders found in places as will later be mentioned, and they are made up 
of very fine, almost impalpable mud. The bulk of the formation consists of black clay shale, 
the typical Ecca shale, crumbling into small pieces by a kind of spheroidal or concretionary 
structure, so that it is hardly possible to get from this shale a piece of a few square inches 
surface without cracks. Alternating with these typical black crumbling shales occur mud- 
stones in well defined layers, the debris of which sometimes afford a good building material. 
*** All the characteristics of the Ecca beds are found again in some parts of the stratified 
Dwyka Conglomerate, and lithologically we might speak of an inter-stratification of these 
Dwyka Conglomerate layers with true Ecca beds.” Moteneraar, 1898, p. 107. 
Blackwelder found large boulders in a regularly stratified slate, and in his 
paper on the glacial origin of a slate in Alaska he writes:. 
“The large size and the variations in both size and composition among the bowlders 
seem incompatible with the hypothesis that the beds have been formed entirely by aqueous 
currents. The subangular yet irregular shapes of the bodies are also more suggestive of 
glacial origin than of any other. There is one feature of the formation which is sufficient, 
however, to prove that even if glacial it is not an ancient deposit of till or moraine, namely, 
the distinct stratification. The shale matrix was evidently accumulated in quiet waters 
where conditions favored the settling of clays and silts in successive horizontal layers.” 
BLACKWELDER, 1907, p. 13. 
Using the regularly banded deposits as a criterion for glaciation it would 
appear almost certain that the slate which Blackwelder described is of glacial 
origin. The age of the slate is still a matter of uncertainty. Ulrich considers 
it of Jurassic age and C. W. Wright that it belongs to the late Carboniferous. 
In 1915 Atwood discovered tillite of early Eocene age in southwestern 
Colorado; he tells me that the mancos shale which underlies the tillite has 
very regular banding. Regarding the matrix of the tillite he writes: 
“The matrix of this lower or yellow till is sand and clay, and it is quite probable that 
most of the clay was derived from the underlying Mancos shale. The sand may have come 
from various formations.” ArtTwoop, 1915, p. 17. 
