SOUTH DAKOTA SCHOOL OF MINES 55 



mon on the sloping surface of the Oreodon beds and is 

 especially characteristic of the rounded hillocks of the 

 Titanotherium beds. This preliminary loosening of the 

 clay, explains perhaps more than any other one feature, the 

 surpassing ease with which the countless tiny channels are 

 formed and how it is that the streams become turbid with 

 every passing shower. 



Any hard layer that may be present tends to resist 

 erosion and this at once initiates surface irregularities. 

 The unconsolidated clays being more rapidly removed, the 

 harder stratum soon stands out in distinct relief and later 

 by undercutting, a precipice develops. Joints often ac- 

 celerate the erosion along certain vertical planes and the 

 result is the development sometimes of cave-like excavations 

 and sometimes of columnar masses. Columns are likely to 

 develop also in connection with hard strata made up of 

 concretionary masses. They are especially abundant in the 

 Protoceras beds, where concretionary masses and jointed 

 sandstones are both abundant. 



Generally the transportation lags perceptibly behind 

 the disintegration and as a consequence a thin fan of sedi- 

 ment clings to the base of every pillar, mound or table. 

 The full extent of these alluvial fans is often not fully dis- 

 cerned. Being formed by the conjoint action of many little 

 streams and made up of excessively fine sediment, their 

 surface slope is low and one readily confuses the alluvial 

 materials with the undisturbed beds on which they lie. 

 As may be readily inferred, there is much transient carry- 

 ing of sediments and much meandering of maturer streams. 

 A single season or even a single freshet often makes im- 

 portant changes in a stream's position and there is a de- 

 cided tendency in the medium sized streams to quickly 

 develop box-like trenches. Cheyenne river and White river 

 are active throughout the year, and during the rainy season 

 they flow in large volume, but the tributary streams coming 

 from the badlands are dry much of the time. Some are able 

 to struggle along in continuous flow for a little while after 

 the rainy reason, but later in most of them little is left but 

 dusty sands and stingy pools of water, the latter clear if 

 strongly alkaline, otherwise turbid to the consistency of 

 mud porridge. 



