212 W. T. LEE AFTON CRATERS OF NEW MEXICO 



the larger crater and the top of the rim is 463 feet. For convenience of 

 description the local names of these depressions may be adopted, the south- 

 ern one being known as the Stehling crater and the northern and larger 

 one as the Kilburn crater. They are essentially alike and the description 

 applies to both, although the observations were made mainly at the larger 

 or Kilburn crater. A sketch map and a section of the Kilburn crater are 

 given in figure 1. 



The sides of the depressions are steep and well exposed. The material 

 beneath the general level of the plain consists of unconsolidated, but well 

 stratified sand, with bedding undisturbed. jSTo fragments of volcanic rock 

 were found in these beds. 



■Above the stratified sand is a sheet of basalt having a maximum thick^ 

 ness of about 15 feet. This nearly surrounds the craters and extends east- 

 ward and northward over a considerable area. The rock contains but few 

 gas cavities, is very compact, and is roughly columnar. 



The circular rim is composed of sand, scoriaceous cinders, fragments 

 of pumice, and angular blocks of basalt. The sand of which the rim is 

 mainly composed is fine, loose, and generally unstratified, although in a 

 few places near the base irregular bedding was noted, the layers sloping 

 outward from the crater and being distinctly unconformable with the 

 underlying sands. The volcanic cinders and fragments of pumice are 

 intimately commingled with the sand and vary in size from that of 

 sand grains to pieces an inch or more in diameter. The blocks of basalt, 

 varying in diameter from a few inches to several feet, are embedded in 

 the sand and cinders of the crater rims, and although they were noted in 

 other parts are most numerous at the northern end of the Kilburn crater. 



Probably the craters were formerly deeper and narrower than they are 

 at present, as is indicated by the dotted lines in the section in figure 1, 

 and have been partly filled by the falling in of the sides. The evidence 

 of this is found in irregular mounds of sand and blocks of basalt fallen 

 from the sides, and in the fact that small portions of the lava sheet are 

 inclined inward, as shown in the section. Further evidence of such fill- 

 ing was obtained in drilling a well in the Kilburn crater by finding a 

 piece of unpetrified wood at a depth of about 100 feet. 



Geographic Features 



In order to understand the significance of these depressions and to 

 find a rational explanation for them, it is necessary to consider some of 

 the surrounding conditions. These have been described in part in a 



