148	THE EMPTY QUARTER
fore, that we have to do with an old forgotten river or rivers
or a delta running down to a sea or bay in this neighbourhood.
We have deep wells, freshwater shells and numerous relics
of human visitation to support the theory, while the existence
of a vast marine bay in the eastern tract of Rub' al Khali
would seem to be borne out by the shallow briny stratum of
water underlying the tract known as Al Khiran, to which we
shall come in due course.
The evidence of altitudes above sea-level does not, how-
ever, suggest a single valley trending northwards from Tu-
wairifa, as Numaila and Bir Fadhil1 both lie at a higher level.
The land surface dips eastwards from Maqainama to the Bir
Fadhil and Numaila tract, which appears to be nearly level in
that longitude and contains the shell-deposit of Shuqqat al
Khalfat, while south of Numaila the land slopes southward
to the valley tract of Tuwairifa. This drop in altitude corres-
ponds with a similar fall from the northern part of Aflaj to
the Maqran neighbourhood, and it may, therefore, be that we
have to do with two valley systems rather than one—the
Wadis of Aflaj proper trending eastward to Bir Fadhil and
Numaila, and the Maqran system flowing to Tuwairifa
parallel to but south of them. This at any rate seems the
most reasonable tentative conclusion for the time being.
As we approached Tuwairifa up the valley the gravel
patches were scored by the usual camel-paths radiating from
any desert well, but I noticed among them three rather
remarkable parallel lines of sinuous tracks, marked more
deeply and running fairly close together. After a while the
sand covered them but they reappeared in the gravel beyond.
My companions agreed that they were remarkable but could
not explain them, and I was left with my own conjecture
that they were possibly portions of an old caravan route
which touched at this watering on its way. The lines I saw
lay south-west and north-east, leading possibly to Maqainama
or more directly to the Aflaj.
JBir Fadhil is 775 feet above sea-level; Numaila 800 feet; and
Tuwairifa only 600 j while Maqainama is 850, These altitudes are, of
course, only approximate, being the result of the computation of my
Aneroid and Boiling Point Thermometer readings.