ON THE PROBABLE GLACIAL ORIGIN OF CERTAIN 
FOLDED SLATES IN SOUTHERN ALASKA 
ELIOT BLACKWELDER 
University of Wisconsin 
In the mountains east of Yakutat Bay, on the southeast coast of 
Alaska, a bowlder-shale terrane of unusual character has been noted 
by Tarr,' and was observed by the writer with some minuteness 
last summer. ‘There is reason to believe that glaciers were instru- 
mental in the formation of this deposit. If the supposition is correct 
a certain novelty attaches to the occurrence because the formation is 
not only old but is highly folded and slightly metamorphosed. 
The rocks are well exposed in the canyons of Moser? and Miller 
Creeks and in the high bench west of them. A terrane, which is 
apparently the same, is that reported by Tarr from the shores of 
Russell Fiord. 
The strata in question are several hundred feet thick and constitute 
a member of the Yakutat series. Typically they are conglomeratic 
shales, or slates, according as secondary cleavage has been developed 
or has not. The body of the rock is a black or dark-gray shale of 
relatively gritty and heterogeneous composition. In many places 
the matrix is definitely stratified, but elsewhere this structure is obscure 
or not visible. 
The pebbles and bowlders in the conglomerate are the features of 
chief interest. Lithologically they include a large variety of rocks, 
some of which are known to occur not far away, while the source 
of others is unknown. Among them are such varieties as greenstone, 
gray limestone, granite, quartzite, graywacke, black slate, and 
flint. In size the bodies range from pebbles to bowlders of large 
dimensions. At least two which were more than fifty feet in 
length were seen imbedded in the shale, while blocks five to ten 
t He mentions shale-conglomerate as a member of the Yakutat series. Bulletin, 
No. 284 (1906), p. 62, U. S. Geol. Surv. 
2 See map in Bull., No. XXI, U.S. Fish Com. 
it J 
