OCR Text |
Show ALLUVIAL CONESâ€"THEIR STRUCTURE. 221 many shiftings, the stream will have swept over a whole semicircle with approximately equal and uniform results. The formation thus built up is an "Alluvial Cone." As we travel over these cones their forms are usually recognized by the eye, though sometimes with difficulty. The slant of the cone (of which more will be said hereafter) is usually quite small, though sometimes very conspicuous. It varies greatly but not capriciously, depending much upon the nature of the materials of which it is composed. Most frequently these cones are so large and so flat, that it is only by very close scrutiny and comparison with surrounding objects that their forms are optically recognized, and many cases occur where we become aware of their true figures and relations only by the use of our pocket instruments. There is one feature which the eye seldom recognizes or even suspects. The profiles are not (even typically) truly conical, but are slightly curved instead of having a rectilinear slope. The}^ are concave upwards, the slope being a little greater near the apex and slightly or sometimes notably diminishing towards the periphery. The slopes near the circumference usually lie between 1° and 2°; those near the apex between 2° and 3J°. The lengths of the radii of the bases often exceed 3 miles, sometimes exceed 4 miles, and seldom fall below 2 miles. Perhaps 3 miles would be a fair average for those found in the valleys of the District of the High Plateaus. So nearly together are the gateways along the mountain and plateau flanks, each having its own alluvial cone, that the cones are confluent laterally; giving rise to a continuous marginal belt along the base of the plateau flanks consisting of alluvial slopes which are sensibly nearly uniform. The conical form of these accumulations is ordinarily tolerably accurate and often remarkably perfect. It is a surprisingly harmonious result of a process which in its elements is apparently irregular, but becomes regular only by averaging the results of its constituents. Not only is the regularity seen in the external form of the cone, but it is found whenever an opportunity occurs to examine its interior structure. This is sometimes revealed to us. In the vicissitudes to which a stream so conditioned is subject it occasionally happens that indirect causes have set it at work cutting into its cone; dissecting it, so to speak, by a deep cut and laying |