Page:The Journal of geology (IA journalofgeology21894univers).pdf/93

This page has been proofread, but needs to be validated.
PHYSICAL GEOGRAPHY IN THE UNIVERSITY.
79

foresight regarding the difficulties that these courses may involve in the process of valley-trenching. The load that they have to carry is relatively light; being only the waste that creeps and washes down the broad constructional slopes, under the guidance of the unconcentrated drainage.

In youth and adolescence, the drainage lines are increased in number and greatly improved in their ability to gather and discharge the rainfall quickly. Numerous little trenches are incised in the broad constructional surface, and the distance that the land waste washes and creeps under the guidance of unconcentrated drainage is much lessened; delay in lakes is decreased; the steep lateral slopes of the young consequent valleys furnish an increasing amount of load to the streams, although they still as a rule have carrying power to spare in their impetuous currents. A good beginning is made in the search for the best location of subsequent streams. As the subsequent streams are better developed in later adolescence, the original broad constructional forms are minutely carved, many subsequent divides are established, the discharge of rainfall is very prompt, and the load of waste that the streams have to carry is notably increased.

In maturity the relief retains much of the intensity of adolescence, and adds thereto a great variety of features. The valley lines are closely adjusted to the structure of the region, this condition having been gained by a delicate and thorough process of natural selection, in which the most suitable drainage lines survive, and the less suitable ones are shortened or extinguished. The impetuosity of youth has disappeared; all the larger streams have developed grades on which their ability to do work is nicely adjusted to the work that they have to do; the lower courses already show signs of age, while the upper twig-like branches are relatively youthful. The whole drainage system is earnestly at work in its task of baseleveling the region, and the forms that the region has assumed bear witness to the close search made by the streams for every available line of effective work.

From this time onward, there is a general fading away of strength and variety, both of forms and activities. The deepen-

This article is issued from Wikisource. The text is available under Creative Commons Attribution-Share Alike 4.0 unless otherwise noted. Additional terms may apply for the media files.