The size and depth of the fuljes varies greatly; some are, as it were, rudimentary only, while others attain a depth of 200 feet and more. The deepest of those I measured proved to be 280 feet, including the sand hill, which may have been 60 feet above the general level of the plain; its width seemed about a quarter of a mile. At the bottom of these deep fuljes, solid ground is reached, and there is generally a stony deposit there, such as I have often noticed in sandy places where water has stood. This bare space is seldom more than a few paces in diameter. I heard of, but did not see, one which contained a well. The wells of Shagik do not stand in a fulj, but in a valley clear of sand, and those of Jobba in a broad circular basin 400 feet below the level of the Nefûd. The fuljes, I have said, run in strings irregularly from east to west, corresponding in this with their individual direction. [244] They are most regularly placed in the neighbourhood of the rocks of Aalem, but their size there is less than either north or south of it. The shape of the fuljes seems unaffected by the solid ground beneath, for at the rocks of Ghota there is a large fulj pierced by the rocks, but which otherwise retains its semi-circular form.
The physical features of the Nefûd, whether they be ridges or mounds or fuljes, appear to be permanent in their character. The red sand of which they are composed is less volatile than the common sand of the desert and, except on the summits of the mounds and ridges, seems little affected by the wind. It is everywhere, except in such positions, sprinkled over with brushwood ghada trees and tufts of grass. The sides of the fuljes especially are well clothed, and this could hardly be the case if they were liable to change with a change of wind. In the Nefûd between Jobba and Igneh I noticed well defined sheep tracks ascending the steep slopes of the fuljes spirally, and these I was assured were by no means recent. Moreover, the levelled track made according to tradition by Abu Zeyd is still discernible in places where cuttings were originally made. Sticks and stones left in the Nefûd by travellers, the bones of camels and even their droppings, remain for years uncovered, and those who cross do so by the knowledge of landmarks constantly the same. I am inclined to think, then, that the Nefûds represent a state of comparative repose in Nature. Either the prevailing winds which heaped them up formerly are less violent now than then, or the fuljes are due to exceptional causes which have not occurred for many years. That wind in some form, and at some time, has been their cause I do not doubt, but the exact method of its action I will not affect to determine. Mr. Blandford, an authority on these subjects, suggests that the fuljes are spaces still unfilled with sand; and if this be so, the strings of fuljes may in reality mark the site of such bare strips as one finds in the intermittent Nefûds. It is conceivable that as the spaces between the sand ridges grew narrower, the wind blocked between them acquired such a rotatory motion as to have thrown bridges of sand across, and so, little by little, filled up all spaces but these. But to me no theory that has been suggested is quite satisfactory. What cause is it that keeps the floors of the deeper fuljes bare; floors so narrow that it would seem a single gale should obliterate them, or even the gradual slipping of the sand slopes above them? There must be some continuous cause to keep these bare. Yet where is the cause now in action sufficient to have heaped up such walls or dug out such pits?
Another strange phenomenon is that of such places as Jobba. There, in the middle of the Nefûd, without apparent reason, the sand is pushed high back on all sides from a low central plain of bare ground three or four miles across. North, south, east, and west the sand rises round it in mountains 400 and 500 feet high, but the plain itself is bare as a threshing-floor. It would seem as if this red sand could not rest in a hollow place, and that the fact of Jobba’s low level alone kept it free. Jobba, if cleared of sand all round, would, I have no doubt, present the same feature as Jôf or Taibetism. It would appear as a basin sunk in the plain, an ancient receptacle of the drainage from Mount Aja. Has it only in recent times been surrounded thus with sand? There is a tradition still extant there of running water.
Jebel Shammar.—A little north of latitude 29°, the Nefûd ceases as suddenly as it began. The stony plain reappears unchanged geologically, but more broken by the proximity of a lofty range of hills, the Jebel Aja. Between these, however, and the sand, there is an interval of at least five miles where the soil is of sandstone, mostly red, the material out of which the Nefûd sand was made, but mixed with a still coarser sand washed down from the granite range. This rises rapidly to the foot of the hills. There, with little preliminary warning, we come upon unmistakeable red granite cropping in huge rounded masses out of the plain, and rising to a height of 1000 and 1500 feet. The shape of these rocks is very fantastic, boulder being set on boulder in enormous pinnacles; and I noticed that many of them were pierced with those round holes one finds in granite. The texture of the rock is coarse, and precisely similar to that of Jebel Musa in the Sinaï peninsula, as is the scanty vegetation with which the wadys are clothed. There are the same thorny acacia, and the wild palm, and the caper plant as there, and I heard of the same animals inhabiting the hills.
The Jebel Aja range has a main direction of E. by N. and W. by S. Of this I am convinced by the observations I was able to take when approaching it from the N.W. The weather was clear and I was able to see its peaks running for many miles in the direction mentioned. With regard to its length I should put it, by the accounts I heard, at something like 100 miles, and its average breadth may possibly be 10 or 15. In this I differ from the German geographers, who give Jebel Aja a direction of N.E. by S.W., on the authority I believe of Wallin. But as they also place Haïl on the southern slope of the hills, a gross error, I do not consider the discrepancy as of any importance.
Of Jebel Selman, I can only speak according to the distant view I had of it. But I should be much surprised to learn that any portion of it passed west of the latitude of Haïl. That portion of it visible from Haïl certainly lies to the S.E., and at an apparent distance of 30 miles, with no indication of its being continued westwards. It is by all accounts of the same rock (red granite) as Jebel Aja.
Between Jebel Selman and the Nefûd lie several isolated hills rising from broken ground. All these are of the sandstone formation of the Hamád, and have no geological connection with Aja or Selman. Such are Jebels Jildiyeh, Yatubb, and Jilfeh, Jildiyeh the tallest having a height of perhaps 3800 feet above the sea, or 300 above Haïl.
Haïl lies due east of the extreme eastern buttress of Jebel Aja, and not south of it as has been supposed. Both it and Kefar, as indeed all the towns and villages of the district, lie in a single broad wady, draining the south-eastern rocks of Aja, and sweeping round them northwards to the Nefûd. The height of Haïl is 3,500 feet above the sea, and the plain rises southwards behind it, almost imperceptibly. The small isolated hills close to the town, belong, I think, geologically to the granite range. The main drainage of the plain south of Haïl would seem to be received by the Wady Hannasy, whose course is north, so that the highest part of the plain is probably between Aja and Selman, and may be as much as 4,000 feet above the sea. This, I take it, is the highest plateau of Arabia—as Aja is its highest mountain, 5000 to 5600 feet,—an all sufficient reason for including Jebel Shammar in the term Nejd or Highland.