838 BULLETIN UNITED STATES GEOLOGICAL SUEVEY. 



of material has taken place at other localities on even a grander scale 

 than here, but we have, in the presence of the monuments, a suggestion 

 that forcibly appeals to the human understanding. Not often do we 

 find a spot where the great activity and the results of erosion are so 

 directly and intelligibly placed before us as here. 



Eegarding the formation of these monuments, we are enabled to 

 gather sufficient data thereupon by observations made on the spot. By 

 primary erosion, due mainly to flowing water, the horizontal distribution 

 of the monument-groups was determined. Channels were cut into the 

 readily yielding material, and thus more or less isolated ridges or groups 

 of the sandstones remained. In part by flowing water, in part by 

 meteoric agents, the soft rock was gradually eroded. Such portions as 

 were most loosely cemented were first attacked, resulting in the forma- 

 tion of excavations of greater or smaller extent. The hard resisting 

 stratum above alluded to as being a red conglomerate acted as a bar- 

 rier to the encroachments of erosion. Protecting, in a great measure, 

 the underlying soft material, it gave way only when its supports event- 

 ually broke down. The constitution of these underlying sandstones is 

 such that they will readily absorb a large quantity of water. By the 

 expansion accomj)anying the freezing of this water, considerable quanti- 

 ties will be " scaled oif ". If this j)rocess continues for a sufficiently great 

 length of time, the weight of the conglomerate will crush its supporting 

 X)ortions, and isolated remnants will mark the direction of a previous 

 continuity. Eaiu, snow, and other atmospheric precipitations will add 

 their share in detaching and removing ]3articles and fragments of the 

 rocks. From such influence the cap-stone will partly protect the col- 

 umn or series of columns supporting it. Erosion by sand can become 

 very aggressive in such instances, provided the wind has ample sway. 

 The sharp particles rapidly eat away ^he more yielding portions, reduc- 

 ing gradually the diameter of the shaft in certain directions. Its 

 repeated action produces a corrugated surface, indicating the locations 

 of the most readily yielding masses. It is due to this influence, proba- 

 bly, that the " neck " of the monument is generally very much narrower 

 than the base. The sand striking against the cap rebounds, and a larger 

 quantity than perhaps otherwise would be the case finds an opportunity 

 to expend its force upon that portion. Totally dependent upon the 

 physical constitution of the eroded rocks are the detail-features they 

 exhibit. In case they are composed of very homogeneous material, the 

 result will be a highly symmetrical product. Inclusions of either 

 harder or softer masses, or a varying density of the rock, will necessarily 

 be made manifest upon erosion. Thus we are enabled to judge, even 

 from the exterior form, as to the general composition of the monument. 



In the course of time, the sustaining column is worn so thin that it 

 can no longer carry the weight of the cap. This falls off, and before 

 long the once stately monument is reduced to a mound of gravel and 

 sand. For a time, the cap may remain comparatively intact, after 



