V.—On the Formation of Ravines by Recent Drift Accumulations

Extract

It is not unusual in drift deposits, banked on hill slopes, to find a deep cut occupied by a very minute stream. These outs or ravines are supposed to be due to the streams; having been excavated by them in the banks of drift. If a ravine of this class is formed in one of the older drifts—namely, the Boulder-clay-drift or the Moraine-drift—it must apparently have been cut by a stream, but much of the drift banked on hill slopes is quite recent; that is formed by meteoric abrasion, and being added to at the present day. In these recent drifts, ravines are of common occurrence, and I would suggest that, instead of the stream cutting its ravine, meteoric abrasion has heaped up the banks, and the only action capable of being done by such a stream, is to keep its channel clear. Such accumulations of drift are very frequent in association with Maums, or connecting gaps across mountain-ranges, and there are always ravines in the banks,—yet by no arrangement of the water-supply, could streams be formed that would have the power to excavate them. In the accompanying sketch of Maumgeeha, Yar-Connaught, there is a maum, below which is a recent drift-bank with a ravine and its accompanying stream. None of the surface drainage of the hills on either side of the maum can get into the stream, it being fed by a spring, while the surface drainage of the mountains flows as represented by the arrows, and yearly the shed from the hills adds to the height of the banks.