tions a pine-oak forest occurs. Further to the west the 
valley becomes narrow and the ridges even higher. 
Covered with pine-oak forests at their summits, these 
ridges cast ‘‘rain shadows’”’ on the bottoms of the valleys 
which consequently are quite dry and have a xerophytic 
vegetation in which maguey, cactus, mesquite, and 
chaparral predominate. Canyon Infiernillo, to which we 
were guided in 1954 by Don Ignacio Guerra, is one of 
those dry canyons containing three caves named 
Romero’s, Valenzuela’s, and Ojo de Agua. These are 
all situated high above the canyon floor at the base of 
limestone cliffs and may have been the mouths of former 
underground rivers that have since become dry and in 
part filled in. These relatively deep tunnel-like caves are 
extremely dry and in prehistoric times would have been 
ideal places for living. Because of their dryness much of 
the refuse and remains of the ancient inhabitants has 
been almost perfectly preserved. 
The Stratigraphy 
All three caves had stratified occupational layers which 
not only revealed a long cultural sequence but also 
yielded many botanical specimens which could be 
brought to bear upon the problems of early agriculture 
and subsistence activities. 
Radiocarbon determinations and correlations of arti- 
facts with those of other sites permit us to recognize six 
more or less distinct cultural phases beginning at about 
2350 B.C. The characteristics of these phases have been 
described in detail elsewhere (MacNeish, 1958). Here 
we need be concerned only with their approximate ages. 
In sequence beginning with the most recent they are: 
San Antonio A.D. 1450-1800 
San Lorenzo A.D. 1050-1450 
Palmillas A.D. 200-800 
[ 35 | 
