768 HEEZEN [chap. 27 



Wtist (1957) concluded that relatively strong bottom currents swept along 

 the continental slope off eastern South America. His calculations indicated 

 a 5-20 cm /sec western boundary current in the South Atlantic. The same range 

 of velocities was recently measured in the North Atlantic western boundary 

 current through the use of neutral floats (Swallow and Worthington, 1957). 



Ocean-bottom photographs of rock outcrops and ripple marks taken on the 

 continental slope show the eroding capability of this current (Northrop and 

 Heezen, 1951; Heezen et al., 1959). Dietrich (1957) and Cooper (1955) have 

 described boluses of cold Norwegian sea-water which flow through the Denmark 

 Strait and descend to the depths of the Atlantic at velocities of 10 to 30 cm/sec. 

 Jarke (1958) has shown the influence of bottom currents on the distribution of 

 sediments on the Iceland-Faeroe Ridge. Strong currents have been calculated 

 from the fine systematic observations made by J. B. Tait (1956) in the Shetland 

 Channel. 



It has been suggested that the boluses of cold water flowing across the 

 northern margins of the Atlantic may be related in some way to the turbidity 

 currents which built or eroded the Northwest Atlantic Mid-Ocean Canyon 

 (Ewing et al, 1953). 



Except where shallow-water benthonic organisms are found incorporated in 

 the deposit, it may be difficult to distinguish in every case the effects of bottom 

 scour from the deposits of turbidity currents. 



More must be learned of the causes and further effect of these strong bottom 

 currents which have denuded the crest of a large part of the Mid- Atlantic Ridge 

 to depths of 2000 fm (Heezen et al., 1959). These currents wear so gradually that 

 trans-Atlantic submarine cables crossing the crest of the ridge often last 25 

 years or more before their armour wires are finaUy worn through. Thus, this 

 incessant motion must never become too violent or the cables would snap more 

 frequently. 



Swallow (1957) has recently made measurements of deep-sea currents in the 

 western Atlantic with neutral buoyant floats. He has found that velocities up to 

 43 cm/sec can be observed in depths as great as 4000 m and that these currents 

 vary in direction and velocity over very short periods of time. Such currents 

 are of such magnitude that the occurrence of ripple marks, scour marks and 

 denuded surfaces in the deep sea is no longer a mystery, and certainly no 

 longer conflicts' with the concepts of modern physical oceanography, 



B. Homogenized History 



Evidence of deep-sea erosion, transportation and deposition by bottom cur- 

 rents comes from the fine studies of the minute, fragile tests of silicious Radio- 

 laria and diatoms found in mid-Pacific abyssal red clays. The long, red-clay 

 cores of the Albatross Expedition were chosen for detailed study since, based on 

 estimates of sedimentation rates, they were thought to contain the undisturbed 

 record of most of the late Tertiary. At the time Riedel (1957) began his in- 

 vestigation of the Radiolaria, he "expected that the assemblages in the tops of 





