466 POPULAR SCIENCE MONTHLY. 



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 op- 

 portunity to observe its internal structure, and to compare it with 

 that observed in the first example. It may happen that the opportu- 

 nity does not exist at this immediate locality, but a little way off a simi- 

 lar deposit occurs, and a beautiful section has been uncovered by the 

 vigorous attacks of a steam shovel. This section has already been re- 

 ferred 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 con- 

 sequently 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 spher- 

 ical, 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 



* In order to obtain this sketch, a survey was made of the delta, and from the informa- 

 tion thus gathered a model was constructed out of clay. The dimensions of the delta are 

 about one thousand by seven hundred feet. 



