NO. 4 SAN JON DISTRICT, NEW MEXICO ROBERTS 21 



third alluviation is indicated by loose and friable alluvium. Fire pits, 

 animal bones, and stone artifacts, late in type, occur in this layer. 

 The preceding stage of arroyo cutting and wind-blown sand, the 

 second erosion interval, is correlated with Pueblo type potsherds that 

 date in the period 1300 to 1540. The second alluviation is charac- 

 terized by an alluvium, humic in places, with irregular, hollow, twig- 

 like limestone concretions (calcareous tubules) in the finer portions. 

 Fire pits, artifacts, and bones from modern bison are found in this 

 level. The first erosion period was marked by arroyos and sand- 

 dune formation, but thus far is not correlated with any evidence of 

 human activity or animal remains. The deposit of the first alluviation 

 contains large limestone concretions and compact humic alluvium 

 in small bodies. Mammoth, sloth, and horse bones have been found 

 in this horizon. Projectile points purportedly of Folsom and Yuma 

 types are supposed to have come from this layer, but the authenticity 

 of this occurrence and the kind of Yuma involved remains to be 

 established. If substantiated, however, and the Yuma proved to be 

 of the Indeterminate type the combination of features would be an 

 important link in correlating the plain deposits and those in the 

 basin on top. 



The deposits in the basin form a complex but definite sequence, 

 one that is in part reproduced in the next gulch to the east. There 

 are 11 stages in all and, as previously suggested, the sequence is 

 related to the filling of an original valley in the escarpment with 

 layers of sand, the "basal sand" of the section. The last or present 

 stage is characterized by the deep ravines and the broad arroyos in 

 the basin and the continuing erosion of today. Preceding this was 

 an interval of alluviation characterized by coarse alluvium that re- 

 mains as terraces. The small barbed arrowheads, potsherds, and 

 grinding stones probably belong to this horizon. The disconformity 

 between this layer and the next lower alluvium is marked by deep, 

 narrow arroyos. The latter were cut into alluvial fill consisting of 

 reddish clay and sand. It is in this horizon that the Yuma (Collateral 

 or Eden Valley) and third type points belong. Broad, shallow 

 arroyos characterize the disconformity between this layer and the 

 underlying alluvium composed of a sequence of clay, red sandier 

 alluvium, and clay. The San Jon point and extinct bison and mam- 

 moth come from this deposit, the lowest level for either animal or 

 human evidence. Below this is a layer of water-laid ash, volcanic 

 in origin, resting on an iron cap and associated clays resulting from 

 the deposition of limonite and clays under wet conditions. The iron 



