Page:Popular Science Monthly Volume 54.djvu/484

466 time the delta is not in vicinity of water. It will be evident to the reader that it differs in no important particular, excepting size, from our little type specimen formed in a pool. Its level top and frontal lobes are to-day nearly as strongly marked as at the time it was made. The reader will have little difficulty in picturing the original conditions of its formation in some ancient lake. This old lake did not endure until the inflowing streams had filled it to a level plain, but for some reason, which it is unnecessary for us to consider, the water was permitted to escape, leaving the delta perched on the valley side. Such deltas are very common, and we find them in all stages, from simple beginnings, as above, to the completed sand plain.

The sand of which our first delta was composed has already been referred to as arranged in horizontal layers. In order to verify our conclusions regarding the origin of this delta, let us seek for an opportunity to observe its internal structure, and to compare it with that observed in the first example. It may happen that the opportunity does not exist at this immediate locality, but a little way off a similar deposit occurs, and a beautiful section has been uncovered by the vigorous attacks of a steam shovel. This section has already been referred to on page 464, as illustrating the structure of the sand layers making up the tiny delta, as well as water deposits in general, and is reproduced here as Fig. 5. The reader will observe in this picture many familiar features common to railroad excavations. The upper part of the geological section thus exposed is somewhat masked by a downfall of sand and loam, and the lower part is also hidden by the same materials. Along the central part, however, the sand and gravel may be seen arranged in horizontal layers of a varying thickness. A close inspection of the uppermost layers will detect a variation in coarseness among the different strata. Such alternations of layers of coarse and fine material are due to differences in the transporting power of the running water that brought the sand and pebbles to their present resting place; the coarse gravel and pebbles were carried by fast-flowing rivers, and the fine sand by streams of less rapidity and consequently less transporting power. Beds of this character ordinarily correspond closely in time with alternating periods of great rainfall or snow melting and the summer seasons. The pebbles of which the coarse layers are composed, as we should expect, are far from spherical, and the operation of gravity on such bodies, as they fall to the floor of a lake or ocean, is to cause them to arrange themselves with their flat surfaces horizontal and parallel to one another. In the