204 GEOLOGY. 



Terrestrial Formations. 



The great warpings and faul tings, and the extensive intrusions 

 and extrusions of lava which marked the close of the Mesozoic era 

 in the western part of North America, appear to have developed lands 

 which were relatively high, in association with tracts which were rela- 

 tively low. The mountain folds, the fault scarps, and the volcanic 

 piles seem to have afforded the elevations necessary for rapid erosion, 

 while the associated valleys and basins and plains furnished lodg- 

 ment areas for such sediments as the streams, descending from the 

 steep slopes above, were unable to carry across tracts of low gradient. 

 Sedimentation on the land was therefore a feature of the Eocene period, 

 as it has been of all subsequent time. Among the accessible forma- 

 tions of this and all later periods, those of terrestrial origin are far 

 more widespread than those of marine origin. 



The terrestrial sedimentation of the Eocene period was probabh 

 comparable to that of the present time, though the western mountains 

 had not then attained their present height. Then as now, temporary 

 and permanent streams were doubtless aggrading their valleys, 

 and building fans and alluvial plains where the appropriate condi- 

 tions were found, while sheet-floods spread debris washed down from 

 the higher lands on the tracts below. The deformative movements 

 which initiated the modern era probably gave rise to basins here and 

 there, in which lakes were formed, and the flows of lava from the unnum- 

 bered vents of the time doubtless sometimes obstructed valleys, pond- 

 ing the streams and giving rise to lakes. Under these conditions, 

 it is probable that much of the debris which was started seaward by 

 the swift waters of the higher lands found lodgment long before it 

 reached the sea, some of it at the bases of steep slopes, some of it on 

 river plains, and some of it in lakes. The wind also made its con- 

 tribution. The result was an inextricable combination of fluvial, 

 pluvial, eolian, and lacustral deposits. 



Terrestrial formations of Eocene age and of fluvial, pluvial, lacus- 

 tral, and eolian origin are widespread throughout the western interior, 

 occurring even in proximity to the western coast. Many of them 

 are of limited extent, while others are spread over great areas. Since 

 the changes which gave rise to the conditions favoring aggradation 

 on the land continued, at least intermittently, during the period, the 



