Johnston — Imbricated Structure in River-gravels. 387 



Art. XXXIII. — Imbricated Structure in River-gravels; 1 

 by W. A. Johnston. 



Gravel deposits formed in river beds and bars fre- 

 quently show a characteristic mode of arrangement of the 

 coarse gravel and shingle. The stones, for the most part 

 dip upstream and overlap or imbricate. This mode of 

 arrangement of the stones was figured and referred to by 

 James Geikie as imbricated structure. 2 It is only rarely 

 described in text books on geology or physiography but 

 has probably been noted by many geologists in the field. 

 Because it is a criterion which can be used occasionally to 

 distinguish marine from non-marine deposits and because 

 of the fewness of such criteria, attention is here directed 

 to it. Furthermore there seems to be a misconception 

 on the part of some geologists as to the character of the 

 structure. 



Imbricated structure was described and its mode of 

 formation explained by G. F. Becker in 1892, who stated 3 : 

 "If a flattened pebble is dropped into a running stream, 

 the water will exert a pressure upon the stone until its 

 inertia is overcome, and during this time the pebble will 

 tend to swing across the current so as to present its 

 greatest area to the pressure. As soon as the resistance 

 due to its inertia is overcome, the pebble will sink through 

 the water as if the fluid were at rest until its edge touches 

 the bottom, and it will then tip downstream until it meets 

 support. . . . Many pebbles thus deposited will, with few 

 exceptions, be inclined downstream and will rest against 

 one another, like overlapping tiles. ' ' This view as to the 

 attitude of the stones agrees with Geikie 's description 

 and with the present writer's observations. The impor- 

 tant points are that the stones tend to dip upstream or, 

 as the placer miners sometimes put it, the up-ends of the 

 stones are the downstream ends or point downstream, 

 and the upstream stones tend to overlap on the down- 

 stream stones. In a recent text-book, however, it is 

 stated 4 that the pebbles in river beds may imbricate up- 



1 Published by permission of the Director, Geological Survey, Canada. 



2 James Geikie: Structural and Field Geology, 1905, p. 311. 



3 Finite Homogeneous Strain, Flow and Eupture of Rocks, Bull. Geol. Soc. 

 Amer., vol. 4, pp. 53-54, 1893. 



4 Lahee, Frederick H., Field Geology, 1916, p. 88. 



