Page:Popular Science Monthly Volume 81.djvu/240

From Wikisource
Jump to navigation Jump to search
This page has been proofread, but needs to be validated.
234
THE POPULAR SCIENCE MONTHLY

PSM V81 D240 Cretacic sandstone and breccia near los cerrillos new mexico.png

Fig. 6. Tilted and Beveled Cretacic Sandstone, overlain by Cemented Horizontal Breccia, near Los Cerrillos. New Mexico.

manifest that notable leveling and lowering has gone on at a rapid rate. Since the San Mateo and Datil plains-surfaces were flooded with lava vast degradation has taken place.

Extensive erosion is everywhere manifest but of peculiar type. There is little of the sharp incision of the plains-surface such as normally characterizes stream-action, especially in a high-lying region having slopes of high gradients. Erosion is of the broad-basin type—wide, flat-bottomed, even plains between abruptly upturned rims against resistant rock masses. As the lava-flows and coulees became more and more numerous the separate basins became divided and smaller, but general lowering of surface went on without interruption. There can be no question but that the lava-capped mesas at varying heights represent former levels of the general plains-surface.

In the outpouring of the molten rock over the surface of the plains the lava-streams naturally flowed down the lowest lines of the plains, but as removal of the weaker beds on either side took place each flow was soon left as an elevation. A particularly instructive instance is shown near the Zuñi pueblo, where an old valley is exposed in a section filled with basalt, the level of the latter in the mesa-face being several hundred feet above the present floor of the plain.

There is further strong evidence of the strictly eolic nature of the landscape sculpturing. These summit plains of the continent are a