834 BULLETIN UNITED STATES GEOLOGICAL SURVEY. 



the water raay carry in suspension. Moving ice and its accompanying 

 mass of detritus manifests the same ultimate achievements. Hard, 

 resisting rocks are smoothed and planed, softer ones are deeply cut into, 

 thus changing the minor orographic features of a region. Dependent, 

 in part, ui)on the physical constitution of the rocks affected, is the action 

 X^roduced by the freezing of water saturating them. In case the con- 

 ditions be favorable, we may find a more or less completely developed 

 system of foliation. Minute fragments are sei)arated from the main 

 rock, and frequently, by a process of regelation within fissures thus 

 formed, large slabs are removed. Certain rocks, less homogeneous than 

 others, absorb a great deal of water, which forces otf innumerable par- 

 ticles upon freezing. In this connection may be mentioned the phenom- 

 enon of "exfoliation". According to the interpretation usually given to 

 this term, it signifies a scaling-ofif of some rocks, dependent upon reach- 

 ing certain temperatures through the action of the sun's rays. Physi- 

 cally this is certainly possible, but I am of the opinion that in reality it 

 does not often occur. Although during the warmer season of the year, 

 rocks exposed to the sun's rays frequently acquire a comparatively high 

 temperature, it seems improbable that this could produce the result of 

 extensive fissures. If we take into consideration the coefficient of expan- 

 sion of the various minerals composing such rocks, and furthermore 

 consider their points of fusion, the suggestion seems still less teiiable. 



Wind erosion, in some highly favored localities, is productive of very 

 striking results. Usually, however, its action is confined to the shap- 

 ing of minor details. Wherever the wind can have full sweep and the 

 sand may find objects upon which to expend its work, there we will 

 soon recognize the peculiar workmanship of this agent. Attacking 

 most rapidly those portions which offer least resistance, the sand will 

 carve out forms which will indicate the physical structure of the eroded 

 material. Exposed stirfaces will be modified in such a way as to denote 

 the prevalent direction of the wind, and so as to furnish an idea of the 

 relative amount of sand utilized in the " blast". 



It would carry us altogether too far were any attempt here made of 

 giving even only the general results of the various kinds of erosion upon 

 different rocks. In the subjoined pages we will have to deal with mainly 

 one class, that produced by mechanical deposition. We shall see that 

 even slight variations in the constitution of this material may be pro- 

 ductive of far-varying results. We have for our consideration a series 

 of forms, referable to several groups, each one of which may be consid- 

 ered as an expression of definite, pre-existing conditions. It shall there- 

 fore be the object of this paper to present them in such a manner as to 

 comprehend their present and eventual form, the materials composing 

 them, and the mode of their formation. 



In order to discuss the material at hand in a somewhat systematic 

 manner, it may appropriately be classified. Among the most prominent 

 forms in Colorado are those that for many years have been known by 



