80 NATURE STUDY REVIEW [9 :3— ^lar., 1913 



— gullies or "washouts" — accomplished by these little temporary 

 streams. A study of a series of gullies, or the same one at inter- 

 vals, reveals that (1) they grow in length by head erosion caused 

 by in-flow of rainwater, (3) they become deeper by erosion on 

 their bottom, (3) they grow wider by side-wash from rains, by 

 slumping and creep, (4) they are V-shaped with steep sides and 

 that the sides become less steep as the gully increases in size, 

 and (5) they have a stream only when it rains. Passing from 

 the largest gully with no stream to the ravine with its little brook, 

 he recognizes the ravine as a grown up gully. He also recognizes 

 that it is different. Although still V-shaped, it has a flat lx)ttom, 

 a permanent and slightly meandering stream, perhaps a small 

 terrace and a flood-plain. How have these changes been pro- 

 duced? is now his problem. He learns that the gully acquired a 

 permanent stream when its bed was eroded deeply enough to secure 

 a constant supply of ground-water, i. c, below the ground-water 

 level. He learns that the current cuts continually on the outside 

 of the bend and makes the stream more crooked, and in doing so 

 it (1) has undercut the sides of the ravine making it wider and 

 flat bottomed, (2) has produced the flood-plain, (3) coupled with 

 erosion on the stream's bed, has sunk the meander belt (belt in 

 which the bends occur) below the level of its surroundings and 

 formed terraces. If the stream has swung to one side of the 

 ravine he finds that side steep and actively eroding and finds the 

 terraces on the opposite side (Fig. 1). But if the stream is in 

 the middle of the ravine he may find terraces on both sides of it. 

 If the stream is a very crooked one he probably can find places 

 where it has cut across the narrow part of the bend and abandoned 

 the meandering channel forming an o.rbozv lake. 



After the above study a visit to the valley of a large river 

 will show it to be like a ravine only on a larger scale, /. e., a gully 

 represents infancy, a ravine middle age, and a valley old age. 

 These terms, of course, are only relative in value and apply to 

 the stage of erosion and not to years. .Are all valleys grown up 

 gullies? and Do all gullies form valleys? are problems that should 

 not be overlooked at this point. That many gullies on a side 

 hill perish as the result of the more rapid growth of one can be 

 demonstrated easily in the field, and that many streams flow 

 awa}- from glaciers, snow fields, and from lakes without ever 

 having occupied gullies will l^e recognized by the student. 



Work of Depositiox. 



Causes of deposition. — During the study of erosion the fact 



