PETROGLYPHS— STEWARD 417 



ascertain their age. Other types of archeological remains often lie 

 in the ground and, if a site has been occupied by different people or 

 in different periods over many years, the relative age of objects may 

 be determined by removing successive strata. This important tech- 

 nique for the historical reconstruction of cultures cannot be used 

 for petroglyphs because they are not stratified. Even the occasional 

 superimposition of one style over another on the same rock has so far 

 yielded few data on relative age. 



There are several means, however, by which the general age may 

 occasionally be ascertained. Geology has sometimes provided im- 

 portant clues. For example, at the Salton Sea, southern California, 

 a few simple, linear petroglyphs occur under layers of travertine, a 

 deposit left on the ancient shore line by the waves of the sea, which 

 was much higher than it is today and filled much of the Imperial 



abed e f 



FiGDEB 3. — Crudely carved pictures of animals, a, turtle ? at Pipestone, Minn. ; h, beaver 

 in Tulare County, Calif. ; c, fish, N. Mex. ; d, bird, W. Va. ; e, bird, Ariz. ; i, bird, 

 N. Mex. 



Valley. These petroglyphs must, therefore, be as old as this inunda- 

 tion, which geologists believe to have occurred between 300 and 1,000 

 years ago. Other petroglyphs on the travertme must, on the other 

 hand, be more recent than the inundation. 



Near Grapevine Canyon, Nevada, there are many complex recti- 

 linear figures on rock surfaces, part of which are now covered by 

 a gravel terrace. The gravel is a stream deposit which, geologists 

 state, was built up several hundred years ago, possibly longer. 



When geologic aspects of this problem have been further exploited, 

 we may expect additional light on the problem of antiquity. Unfor- 

 tunately, however, geologic estimates of age are always broad and 

 can seldom fix dates with the precision required to relate petroglyphs 

 to other types of archeological remains having known antiquity. 



The degree to which petroglyphs have faded out through weather- 

 ing has been examined with considerable care. This is one of the 

 least reliable measures of age, for the mere appearance of antiquity 

 is no proof whatever of great age. The writer has seen dates carved 

 on rocks not over 10 years ago which have weathered almost beyond 

 recognition, whereas definitely pre-Columbian petroglyphs close by 

 are still bright and fresh. The time required for weather to oblit- 

 erate a petroglyph depends upon the kind of rock on which it is 



