268 GEOLOGY OF THE DENVER BASIN. 



surface. The minor details of sculpture, owing to the different materials 

 in which the carving is effected and the diminished force of the degrading 

 agencies, necessarily present different features. The characteristic resist- 

 ance of loessial material to lateral erosion, and its readiness to submit to 

 vertical corrasion, are admirably adapted to produce vertical lines of relief. 

 I )n Sand Creek, cliffs of this material show vertical faces 30 to 40 feet 

 high, and oppose a resistance to weathering ajid the lateral corrasion of 

 the stream comparable to that of an indurated sandstone. 



The surface of the loess, except near the streams, where the edges are 

 drained by finished drainage svstems, is covered by a series of low ridges 

 and knolls, and of shallow troughs and circular basins varying greatly in 

 diameter and depth. No apparent system can be noticed in the arrange- 

 ment of these features, nor do any of the ordinary agencies of erosion appear 

 to explain their peculiar formation. Something of a similar nature has 

 recently been noticed in the seolian loess of Asia, where it seems to have 

 been formed by subsidence due to the removal of the lower layers by the 

 action of underground streams. AVhere the bottom of a basin is formed 

 in semi-indurated loess, the fine silt brought down from the surrounding 

 slopes will form a sufficiently impervious layer to permit in dry areas the 

 existence of small playa ponds, and in the rain-belt oountry and in 

 irrigated districts of permanent ponds. The playas are of great value to 

 stockmen in lessening the distance that animals must travel for water and 

 thereby extending the amount of available range. The poor homesteader 

 is saved, by proximity to a playa, the considerable expense of sinking a 

 well for the purpose of watering his stock, and is only obliged to bring a 

 i'vw barrels of water for household purposes from the well of some more 

 fortunate neighbor. 



iEolian agencies have accomplished important work in finishing the 

 minutiae of the surface features. The patches of cactus (Opuntia) and the 

 mats of the moss-like buffalo grass afford poor protection to the violent 

 winds of the region. The loosened silt, resembling the surface on which 

 it is deposited, can not lie readily detected after a storm has moistened the 

 soil. How far it assimilates with the surface, and to what extent it is 

 washed into the streams, is a problem for future solution. At the foot of 



