=Water and wind gaps.=—In the Allegheny plateau rivers cross, the range of harder rocks in deep mountain narrows which upon the horizon appear as gateways through the barrier of the mountain wall. Such gateways are sometimes referred to as “water gaps”, of which the Delaware Water Gap is perhaps the best known example, though the Potomac crosses the Blue Ridge at the historic Harper’s Ferry through a similar portal. The valley of the tributary Shenandoah has been the scene of an interesting episode in the struggle of rival streams which is typical of others in the same upland region. The records which may be made out from the landscapes show clearly that in an earlier but recent period, when the general surface stood at a higher level which has been called the Kittatinny Plain, the younger Potomac of that time and a younger but larger ancestor of Beaverdam Creek each crossed the Blue Ridge of the time through similar water gaps (Fig. 185, map, and Fig. 186). The Potomac of that time was, however, the more deeply intrenched, and possessing an advantage in slope it was able to advance the divide at the head of its tributary, the Shenandoah, into the territory of Beaverdam Creek. Thus the beheading of the Beaverdam by the Shenandoah was accomplished (Fig. 185, second map) and its upper waters annexed to the Potomac system. With the subsequent lowering of the general level of the country which yielded the present Shenandoah Plain, the former water gap of Beaverdam Creek was abandoned of its stream at a high level in the range. Known as Snickers Gap, it may serve as a type of the “wind gaps” of similar origin which are not altogether uncommon in the Appalachian Mountain system (Fig. 186). [Illustration: FIG. 186.—Section to illustrate the history of Snickers Gap.] =Character profiles.=—For humid regions the landscapes possess characters which, speaking broadly, depend upon the stage of the erosion cycle. For the earliest stages the straight line enters as almost the only element in the design; as the cycle advances to adolescence the rounded forms begin to replace the angles of the immature stages, and with full maturity the lines of beauty alone are characteristic. As this critical stage is passed irregularity of feature and ever more flattened curves are found to correspond to the decline of the river’s vital energies. There are thus marks of senility in the work of rivers (Fig. 187). [Illustration: FIG. 187.—Character profiles of landscapes shaped by stream erosion in humid climates.] READING REFERENCES FOR CHAPTERS XII AND XIII General:— SIR JOHN PLAYFAIR. Illustrations of the Huttonian Theory of the Earth. Edinburgh, 1802, pp. 350-371. J. W. POWELL. Exploration of the Colorado River of the West and its Tributaries. Washington, 1875, pp. 149-214. G. K. GILBERT. Report on the Geology of the Henry Mountains. Washington, 1877, pp. 99-150. (A classic upon the work of rivers.)