Ij] LANDSCAPES AND VEGETATION 543 



is appreciated when, as is often the case, potentialities for future 

 development emerge. As an example of an easily recognized feature 

 we may consider an esker, the elongated ridge of gravelly material 

 that is often left after the disappearance of an ice-sheet. The presence 

 of a well-formed esker indicates not only that the area was glaciated 

 in the past but also that the glaciation was of the over-all, ' continen- 

 tal ' type and that the ice was locally stagnant for a considerable time. 

 Knowing that an esker is formed of assorted gravel or sand, we 

 can tell at a glance that it will afford an abundance of good building 

 or road-making material as well as good drainage, though there will 

 probably be plentiful water and fertile outwash plains nearby. An 

 ecologist could probably go farther, and tell us, for example, even if 

 the esker is covered with forest, whether it could be converted into 

 productive pasturage. 



Landforms AND Plant Life 



It will be appropriate at this point to consider in turn the various 

 destructional landforms that are most common or important and, 

 at the same time, their more characteristic forms of plant life. This 

 consideration may conveniently be given under the headings of the 

 chief agents of erosion, which are streams, glaciers, ground-water, 

 winds, and waves and currents. Each resultant structural group may 

 then be subdivided into (a) erosional, (b) depositional, and (c) 

 residual, landforms or minor features. As already suggested above, 

 the vegetational features of constructional landforms such as plains, 

 plateaux, and mountains tend to be too general (or, in the case of 

 mountains, comparably variable in different instances) to be distinc- 

 tive and pertinent in this connection, being moreover already con- 

 sidered in our treatment of the main climatic belts of the world. 



Erosional features made by streams include, besides peneplanes 

 which are too generalized for specific vegetational characterization, 

 various kinds of valleys and gorges. These, being more sheltered 

 and often much lower than the surrounding uplands, tend to support 

 more luxuriant vegetation. Indeed many plants, for one reason 

 or another, are restricted in a particular region to valley sides or 

 eroded bottoms, and much the same may be the case with whole 

 plant communities — including some of the most important to man- 

 kind. Thus, near the northern limit of arborescent growth, trees 

 are largely or entirely restricted to valleys — for example in northern 

 Alaska, Labrador, and Lapland — as they are also in many arid regions 



