STRUCTURES DEVELOPED IN MARINE BEDS 909 



any extensive series like the Catskill beds of southwestern New York^ in 

 whicli argillaceous sandstones and sandy shales are conspicuous elements 

 and pure limestones wanting, might properly be suspected of having orig- 

 inated in fresh water. 



Structures developed in marine Beds 



The rapidity with which deposition of fine suspended matter proceeds 

 in marifie water under favorable conditions of supply of sediment leads 

 to the phenomena of vertical currents, which in the case of fine muds may 

 leave a characteristic and distinctive imprint on the material laid down 

 which will distinguish it from fresh-water sediments.^ These structures, 



Figure 3. — Pit and M&und structures developed on the Surface of ivater-laid Sediment 



hy vertical Currents 



About natural size 



which may be termed pit and mound structures, may be developed experi- 

 mentally by adding to a glass containing an aqueous mixture of fine clay 

 a pinch of salt. The flocculation of the clay which follows the addition 

 of the salt will be accompanied by the development of numerous vertical 

 currents. These currents develop chimney-like channels through the 



^ E, M. Kindle : Small pit and mound structures developed during sedimentation. Geol. 

 Magazine, vol. iii, Dec, 6, lOlG, pp. 542-.547, pi. xiii. 



>foTE. — In a review of this paper (Ind. Alum. Mag., vol. 4, p. 429, 1917), Prof. B. R. 

 Cummings reports having observed numerous examples of these structures in the Rich- 

 mond formation of southern Indiana. 



