244 PRINCIPLES OF STRATIGRAPHY 



Gastropods have been noticed with transversally arranged frag- 

 ments, which apparently were arrested by the immovable shell, on 

 the east side, and with a drift ridge of longitudinally arranged 

 fragments on the west side." (Ruedemann-59: j^'o-j^'/.) Since 

 the general direction of the great ocean currents seems to have been 

 to the northeast, this indication of a southwestward flow estab- 

 lishes a secondary recurving current of the type common at the 

 present time. 



Depth of Current Action. The depth of current action can 

 often be ascertained by the scouring which it accomplishes on the 

 bottom, sweeping off all fine sediment, and leaving a "hard bottom." 

 T. M. Reade has recorded such bottoms between Gran Canaria and 

 Teneriffe in the Canary Islands in depths of 2,000 meters. (T. M. 

 Reade-56.) This is, however, to be regarded as more properly the 

 work of a tidal current. In the Florida straits, at depths of 160 to 

 550 meters, the bottom is kept clean, the fine mud being swept be- 

 yond the edge of the Pourtales Plateau, which consists of recent 

 organic material consolidated into a hard breccia. The bottom of 

 the Straits of Gibraltar is swept clean and smooth through the out- 

 ward flowing bottom current. 



Lake Currents. 



Lakes, owing to their usually small size, are not influenced in 

 the same manner by the primary current-producing agents as are 

 the larger water bodies. It is true that on the large lakes, like those 

 of North America, longshore currents, due to prevailing winds, 

 are of considerable significance, not only in navigation, but in the 

 transportation of material along the beach, and the building of 

 bars, sandspits, etc. In general, however, the movements of lake 

 waters induced by wind partake of the nature of a vortical circu- 

 lation. The wind blowing steadily across the surface of a lake 

 forces the water to the opposite shore, where it sinks, while the com- 

 pensatory streams behind the wind rise from below. This produces 

 an under current flowing in the opposite direction from that on the 

 surface, and at a depth depending on the temperature, density, etc., 

 of the water and the configuration of the basin. (Forel-24: 81-83.) 



River Currents. 



Rivers are currents of water confined between banks of rock or 

 soil, and they differ from currents in water bodies in that typically 

 their movement is due to gravity alone. While the whole mass of 



