544 PRINCIPLES OF STRATIGRAPHY 



forms the surface material into a pasty mass, and this under the 

 influence of gravity will move slowly down even gentle slopes, the 

 movement being a flowage at a rate much exceeding that of the 

 slow, subaerial creep. Along the valleys mud streams or mud gla- 

 ciers are formed, the movement of which may be prolonged during 

 a considerable period of time. Such mud streams consist of ma- 

 terial of all sizes from the finest grains to huge, angular blocks. 

 The rapidity of movement of the mud streams of Bear Island is 

 shown by the fact that vegetation rarely gets a foothold on these 

 masses of debris. The width of the mud streams of Bear Island 

 ranges up to 35 meters and the depth to about 2 meters. 



The "stone rivers" of the Falkland Islands form other examples 

 of such atmoclastic accumulations. (Darwin-ii.) Valleys here 

 are filled by gray masses of rock debris, with a width of several 

 hundred meters to several kilometers, and having from a distance 

 a glacier-like aspect. Beginning in the uplands, they descend to the 

 low country, several streams uniting, until finally the entire mass 

 debouches into the sea. The streams include vast accumulations of 

 quartzite blocks of irregular form, though parallelepipedal forms 

 predominate. In length they vary from i/^ to 7 meters, the width 

 being half as great, while their thickness depends on that of the 

 original beds of the parent rock from which they are derived. 

 These blocks are angular and are piled one above another in an ir- 

 regular manner. It has been held that these streams, which are now 

 mostly covered by vegetation, came into existence when the Falk- 

 land Islands were experiencing a rougher climate, comparable to 

 some extent to that of Bear Island to-day. The original arrange- 

 ment of the strata into hard quartzite beds enclosed in soft layers is 

 considered as having a fundamental influence on the formation of 

 these rock streams, this arrangement being the usual one where such 

 streams are well developed. 



Stone glaciers, a modification of the rock stream phenomenon, 

 have been described from Alaska (Capps-7). They vary in length 

 from one to five kilometers, and in width from 100 to 500 meters. 

 The angle of surface slope varies from 9 to 18 degrees, and the size 

 of the rock fragments averages 20 cm. for porphyry, but more for 

 diorite and limestones and less for slates. Blocks several feet in 

 diameter also occur. In form these stone glaciers resemble true gla- 

 ciers, being thickest in the center, where they also show evidence of 

 more rapid motion. Frontally these glaciers end in a face up to 

 30 meters in height and having a slope generally of 35°, the maxi- 

 mum slope of material of such coarseness. The whole aspect of 

 the mass suggests motion. 



