S8 BUREAU OF AMERICAN ETHNOLOGY [bull. 54 



the streams in that direction, causing them to undercut the canyon 

 walls and thus produce cliffs on the northern side. Locally in the 

 Frijoles and some of the other canyons such undercutting is going on 

 now for very short distances. However, no evidence of such tilting 

 has been discovered here. 



With a hard zone above, and a thick, very soft bed of tufa below, 

 such as exists in this region, any or all of the forces suggested would 

 operate, perhaps, as causes in the production of more pronounced 

 cliffs on one side than on the other, but all of them combined would 

 not fully explain the present condition. With the same hard zone 

 underlaid by soft material on the southern side of each canyon, surely 

 high cliffs must form on that side unless some force not operative 

 on the northern side is engaged in breaking down all incipient cliffs 

 and terraces. That force is frost, one of the most effective causes of 

 disintegration in all regions where water finds its way into crevices of 

 the rocks and there freezes. Water in freezing expands about one- 

 eleventh of its bulk or one thirty-fifth of its linear extent, exerting a 

 force of a great many tons to the square foot. As snow occurs and 

 water freezes in the region under discussion, it seems easy to account 

 for the absence of the cliff on the southern side — the side facing 

 northward. The tufa is deeply fissured wherever exposed, on both 

 sides. Whenever the weathering of the soft material leaves one of the 

 hard zones standing out on the southern side as an incipient cliff or 

 terrace, water from winter storms finds its way into the crevices and 

 freezes, thus prying off blocks of tufa and destroying all cliffs and 

 terraces in their youth, leaving a fairly uniform slope. It is not 

 difficult to understand why this does not also occur on the northern 

 side. The northern wall, facing the south or southwest, receives the 

 rays of the winter's sun at their most potent angle, and the cold is not 

 so intense but that the rocks are thus kept well warmed, while the 

 opposite wall of the canyon receives comparatively little heat during 

 the coldest "weather. Hence snow falling on the northern wall is 

 soon melted and the water evaporates or runs off without freezing. 

 Even in the most 'severe weather in northern Colorado, up to an 

 altitude of 12,000 to 13,000 feet, where the snowfall is heavy, most 

 of the southerly slopes are kept bare of snow through a great part of 

 the winter. Much more would this be true at the Frijoles. Having 

 reached this conclusion, the writer questioned Indians and whites 

 who are familiar with the region, and they agreed that snow seldom 

 remains on the northern side more than a few hours, but often 

 remains on the southern side for weeks at a time. As differential 

 erosion in a formation such as this would produce cliffs in the absence 

 of any force tending to tear them down, the presence of a force which 

 would tear down the cliff's on one side and not operate on the other 



