344 



HEEZEN AND LAUGHTON 



[chap. 14 



The best explored abyssal gap (Figs. 23 and 24) and associated system of 

 inteqjlain channels is that connecting the southwest corner of the Biscay 

 Abyssal Plain with the northern part of the Iberia Plain (Laughton, 1960). 

 This has been called by Heezen et al. (1959) the Theta Gap. There is a difference 

 in level between the two plains of about 100 fm which can supply enough 

 kinetic energy that turbidity currents flo\\'ing through Theta Gap become 

 actively erosive. On the Biscay Plain a dendritic system of feeding channels 



I3°30' 

 I? 



44 30' 



44°00' 



-1330' 



I3°30' 



I3t)0' 



1 2° 30' 



1?°0C' 



11° 30' 



1 1°00' 





X 





,-' — ^' 



[' / / 



I3°00' 



12 30' 



I2°00' 



.n the 



Southwest Biscoy 

 abyssal plain 





ObOvf ?700 'm 



Ai*s -I :honr>«ls ~- — — — . 



Sounding line* 



H4ly areo& 



CfB|> 



44" 30' 



44 00 



43°30' 



1 1 30 



1 1°00' 



Fig. 25. Feeding channels in the southwestern part of the Biscay Abyssal Plain. (After 

 Laughton, 1960.) 



(Figs. 25 and 26) converge on an interplain channel which passes through the gap. 

 In places these feeding channels are box-shaped, 2 or 3 mi wide and 10 or 20 fm 

 deep : in other places they are V- or U-shaped. They join finally to form one 

 broad V-shaped channel which drops quite suddenly more than 80 fm into 

 the narrow, vertical-sided gorge of the interplain channel. Nearly all the change 

 of level between the plains takes place within less than half a mile. 



Thereafter the interplain channel winds through the abyssal gap exhibiting 

 the characteristics of a meandering river, the deef)est point swinging to the 

 outside of a bend and the depth varying according to the width of the channel. 

 It is joined half way by another branch of the channel system, divides around 

 a " middle ground " hill and finally joins the Iberia Plain on a broad front. 

 There is no sign of discrete channels on the Iberia Plain. 



The feeding channels of the Biscay Plain extend some 80 mi from the head 



