182 STUDIES FOR STUDENTS 



proximity of the shore. The subaqueous origin of the breccia is 

 confirmed by Van Tuyl, 1 but only in part. The first period of dis- 

 turbance was one in which, under violent wave-action, mounds of 

 shoal breccia were produced. The major disturbances, however, 

 are of later date and gave rise to tectonic breccias associated with 

 mashing, folds, and overthrust faults. 



Tide-glacier breccia. — -Tide glaciers, laying their loads on sea- 

 bottom, give rise to breccias which prove their parentage by faceted 

 and scored pebbles of a considerable variety of rocks and by sub- 

 jacent disturbed sedimentary deposits or glacier pavements 

 where the ice has overridden the sea-floor. Associated stratified 

 beds with littoral fossils prove the breccia submarine. Icebergs 

 detached from the glacier front extend the formation seaward in 

 an ice-raft breccia, with a lessening proportion of morainal stuff. 



The Chaix Hills, described by Russell, 2 are carved from an 

 up tilted block 4,000 or 5,000 feet thick, composed of morainal 

 material and sea-clays. The fragments of this breccia are sporadic 

 throughout the terrane from base to summit. They are both 

 angular and rounded and reach a diameter of some 8 feet. Litho- 

 logically they are as various as are the bowlders of the moraines 

 of the living glaciers of the encircling mountains. Sea-shells of 

 living species are numerous in the finer portions, which are largely 

 made of glacier silts. 



Glide breccias. — The sediments of the sea-floor are subject to 

 slow and rapid gravitational movements, comprehensively termed 

 glides, which deform, shatter, and brecciate the involved strata. 

 Glides may be expected to affect the steeper slopes, such as the 

 sides of submarine channels, the front of deltas, and the edges of 

 continental shelves. They are known to have taken place on 

 slopes as low as about three degrees. The mobility of marine 

 deposits is increased by permanent saturation and frequently by 

 lack of cementation. 



Subaqueous glides may be classified according to their chief 

 precipitory causes as overload glides, earthquake glides, and deforma- 

 tion glides. 



1 F. M. Van Tuyl, "Brecciation Effects in the St. Louis Limestone," Bull. Geol. 

 Soc. Am., XXVII, 122-24. 



2 1. C. Russell, "Second Expedition to Mt. St. Elias," U.S. Geol. Surv., 13th 

 Ann. Rept., pp. 24-26. 



