(g) Eruptive Basalts.
(a) Surface Deposits and Boulder-beds.—Next to the Tufa crust already described, which extends over almost the entire plain of Marocco, perhaps the most remarkable feature in the physical geology of the country is the enormous deposit of boulders that occurs in the lateral valleys, and flanks the great chain on its confines with the plain. Of marine drift there is not a trace; and alluvial drift and valley gravels are very limited in their distribution, being confined to the borders of a few insignificant rivers that intersect 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 Marocco to the Atlas that the great boulder-beds came under our notice, first in a valley leading up from Mesfioua to Tasseremout, 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 decide that they were other than stream-borne masses from the higher ground. From Tasseremout we turned west, and at the
Fig. 5.
Boulder-mounds, skirting Atlas Plateau Escarpment. (Section.)
mouth of a second valley, 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 NW. of Tasseremout, 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 cliff-like face; but below it the great boulder-beds (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 escarpment in one direction, and in the other rolling away in great wave-like ridges and undulating sheets, which terminate at a well-marked line of demarcation, just where the level portion of the plain commences. I measured by aneroid the height of these mounds; and at one point their summit was 3,950 feet above the sea-level, from which they spread down uninterruptedly to the edge of the plain nearly 2,000 feet below. They bear a striking resemblance to the glacial ridges or escars between Edinburgh and Perth; their mound-like structure is distinctly visible from the city of Marocco, twenty-five miles off, 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
Fig. 6.
Boulder-mounds, skirting Atlas Plateau Escarpment.
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.