THE MOON 353 



becomes a crater by the conversion of its contents into 

 snow flakes and the settling of the latter on the brims. 



The very largest craters on the moon are, literally, 

 the maria themselves, for here was the chief source of the 

 snow supply. Around their borders on all sides, there- 

 fore, stand high snow cliffs, some of them very precipi- 

 tous ; while at other places there are terraces, or perhaps 

 gentle slopes masking the elevated character of the mesas 

 or plateaus at their backs. These differences are due to 

 the varying natural depth of the ocean bed at the shore 

 line. If the offset is abrupt, the gathering water from 

 the summer thaws washes and undermines the base of the 

 glacial cliffs, keeping them sharply trimmed, whereas if 

 there is a natural beach, a different order prevails. 



When snow is piled too high, its own weight causes 

 it to collapse or telescope upon itself, resulting in two 

 things ; first, the top layer flattens out and overhangs the 

 base, forming a lip, which in time loses its balance and 

 falls, forming a terrace or talus; this event being per- 

 haps followed by another similar avalanche farther on, 

 provided the pile be sufficiently deep ; and, secondly, the 

 increased weight finally reaches an extreme limit, where 

 It liquefies and squashes out the undermost layer into 

 the flanking pools. These two processes, major though 

 they are, are yet beyond the range of our telescopes to 

 reveal and rest solely upon deduction. Taken in connec- 

 tion with the continuous repair work being performed by 

 the ever-rising snow, they suffice to keep the lunar struc- 

 tures looking remarkably clean-cut and permanent, a cir- 

 cumstance which has led astronomers into the erroneous 

 belief that there is an absence of " weathering" on the 

 moon. 



When the process of glaciation originally began, the 

 lunar atmosphere was, of course, very much denser than 

 it is now, because the snow-that- was-to-be had not yet ab- 

 sorbed it. As a consequence, the meteorological condi- 

 tions at the outset were much different from now, for 

 whereas the winds doubtless then played a strong role 

 in scattering the snows far and wide over the continental 



