92 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jan, 10, 



already described, which extends over almost the entire plain of 

 Morocco, perhaps the most remarkable feature in the physical 

 geology of the country is the enormous deposit of boiilders that oc- 

 curs in the lateral valleys and flanks the great chain on its con- 

 fines with the plain. Of marine drift there is not a trace ; and al- 

 luvial drift and valley-gravels are very limited in their distribution, 

 being confined to the borders of a few insignificant rivers that in- 

 tersect the plain and the localities of occasional waterflows ; but as 

 soon as the flanks of the Atlas are reached, new and distinct drift- 

 phenomena present themselves. It was on our second day's journey 

 from Morocco to the Atlas that the great boulder-beds came under 

 our notice, first in a valley leading up from Mesfeua to Tassema- 

 rout, as scattered blocks of Red Sandstone, remarkable for their 

 large average size, many of them of from ten to twenty cubic yards ; 

 but here the method of their disposition scarcely enabled us to de- 

 cide that they were other than stream-borne masses from the higher 

 ground. From Tassemarout we turned west, and at the mouth of 

 a second vaUey, two miles from the village, suddenly came upon a 

 huge development of these Red Sandstone boulder-beds as great 

 ridge-like and very symmetrical masses with terminal faces three or 

 four hundred feet high, and, like the more scattered blocks N.W. of 

 Tassemarout, intermixed with but a very small proportion of fine 

 matter. From this valley we turned out northwards, skirting the 

 escarpment facing the plain ; and for more than ten miles no lateral 

 valley breaks into the clifi-like face ; but below it the great boulder- 

 beds (PI. III. figs. 5, 6) still occur in huge masses not resting directly 

 against the escarpment, but as isolated mounds two or three hundred 

 feet in advance, sloping down towards the escai-pment in one direc- 

 tion, and in the other roUing away in great wave-like ridges and un- 

 dulating sheets, which terminate at a well-marked line of demarca- 

 tion, just where the level portion of the plain commences. I mea- 

 sured by aneroid the height of these mounds ; and at one point their 

 summit was 3950 feet above the sea-level, from which they spread 

 down uninterruptedly to the edge of the plain nearly 2000 feet below. 

 They bear a striking resemblance to the glacial ridges or escars 

 between Edinburgh and Perth ; their mound-like structure is di- 

 stinctly visible from the city of Morocco, 25 mUes ofi", appearing like 

 a row of pyramidal tali resting against the face of the escarpment 

 as though they had been cast down from its edge on to the plain. 

 The internal structure of the mounds also suggests such a deviation 

 from the disposition of the boulders in layers sloping away from 

 the escarpment towards the plain ; and on a nearer approach it is 

 seen that the individual mounds are not connected with channels 

 or valleys breaking through the escarpment. 



The depression between the escarpment and the drift-mounds is 

 a remarkable feature, and suggests an entire change of conditions 

 since the boulder-beds were deposited. If they are a mere subaerial 

 talus, they should rest directly against the cliff face, and the valley 

 of separation must have been formed after the accumulation had 

 ceased ; and yet no satisfactory reason can be assigned for such ces- 

 sation, if rain and river-action were the only operating causes. 



