CRETACEOUS SYSTEM. 



35 



beds" of Mr. Griesbach are Lower Cretaceous and may correspond 

 approximately to the Barremien stage ; the Red Grit series must 

 therefore be younger. 



Cretaceous System. 



This comprises by far the most widely distributed group of rocks 

 in Afghanistan to the north of the Koh-i-Baba and the Hindu Kush. 

 To anyone looking northward from either of these mountain ranges 

 the whole country appears to consist of gently undulating downs with 

 here and there a small limestone scarp, and it is only when one begins 

 to cross the downs that this appearance is found to be highly illusory, 

 for at every few miles they are cut up by deep valleys, from a few 

 hundred yards to a mile in width and as much as 2,000 or 3,000 feet 

 deep, running approximately east and west and connected with one 

 another by narrow precipitous gorges. 



From the Koh-i-Baba and the Hindu Kush northwards to the plains 

 of Afghan Turkistan, the whole area was formerly covered by a sheet 

 of Upper Cretaceous limestone, which was deposited unconformably on 

 all older formations. At first sight, this appears to lie in almost hori- 

 zontal beds and we conclude that there has been but little post-Creta- 

 ceous crust-movement; but when we descend from the undulating 

 plateaux into the valleys we se a at once that this first impression is 

 quite erroneous and that the horizontality of the beds is to be ascribed 

 to the considerable folding that they have undergone. This apparent 

 paradox arises from the type of structure — the recumbent fold — which 

 persists throughout the whole area. Pig. 1 (p. 3) shows diagram- 

 matically the prevailing tectonic conditions, which are also exemplified 

 in Plates 2, 3 and 6. It will be seen that the valleys are cut out 

 along the broken anticlinal crebts, whilst the high intervening plateaux 

 are formed by a limb of the recumbent fold. Sometimes the inversion 

 is combined with overthrust along: the middle limb of the fold as at 

 Pasht-i-Safed in Kahmard (fig. 2, p. 4). 



P % 



