1 64 STUDIES FOR STUDENTS 



An example of a talus breccia is described by Wilson 1 as occur- 

 ring at the base of a cliff of Keweenawan sandstones covered by 

 a sheet of ancient diabase. 



Rock-glacier breccia. — A subspecies of talus breccia is that pro- 

 duced by rock glaciers, talus glaciers, or rock-streams, as they are 

 variously known. 2 Talus may be very rapidly produced where the 

 wedgework of frost is especially efficient, as in cold climates, on 

 the lofty walls of cirques composed of rock that favors fragmenta- 

 tion by close jointing, or other structures. Under the urge of its 

 own weight and that of the expansion of interstitial ice talus 

 creeps forward from the feeding cliff in long tongues of waste which 

 in shape somewhat resemble glaciers. Rock-glacier breccia differs 

 from talus breccia only in its greater extension normal to the cliff, 

 in the movement of the material to greater distances and with 

 very gentle slopes, and in the fact that unequiaxed blocks may be 

 expected to be found set at all angles, owing to the movement of 

 material en masse. 



The famous limestone breccias of Gibraltar, described by Ramsay 

 and Geikie, 3 are attributed to this class. These breccias occupy 

 wide tracts about the base of the Rock of Gibraltar and reach a 

 thickness of at least ioo feet. Their slope in places does not 

 exceed two or three degrees. The fragments are almost invariably 

 quite angular. They vary in size from grit up to blocks 12 feet 

 or more in diameter, and are distributed without regard to size 

 and shape. They are no larger at the base of the cliffs than on the 

 outskirts of the formation. The matrix is earthy and cements 

 the breccia into firm rock. The authors cited attribute frag- 

 mentation to the work of frost under far severer climatic conditions 

 than now obtain. The assemblage of the breccia by gravity in 

 ordinary talus is negatived by the extent and slope and apparently 

 by the set of the fragments. The angularity of the fragments, their 

 size, and their lack of sorting preclude the theory of transportation 



1 A. W. G. Wilson, "Trap Sheets of the Lake Nipigon Basin," Bull. Geol. Soc. 

 Am., XX, 207-9. 



2 Whitman Cross and Ernest Howe, U. S. Geol. Surv., Geol. Atlas, Silverton 

 Folio, p. 25; S. R. Capps, "Rock Glaciers in Alaska," Jour. Geol., XVIII, 359-75. 



3 A. C. Ramsay and James Geikie, Quar. Jour. Geol. Soc. London, XXXIV, 

 505-41, 1878. 



