478 PROCEEDINGS OF THE NATIONAL MUSEUM VOL. 92 
OCCURRENCE AND PRESERVATION OF THE MATERIAL 
The principal sites in the San Pedro Valley from which collections 
were made have come to be known as the Benson and Curtis ranch 
localities (pl. 42). The Benson occurrence is about 2 miles south of 
the town of Benson to the west of the San Pedro River. The location 
is given by Gidley as sec. 22 (this appears to be sec. 21), T. 17 S., 
R. 20 E. (1926, p. 85), and all the Benson rodent and lagomorph 
materials collected are stated by him (1922, p. 119) to have come 
from one spot, a fossil bone quarry. The bone from here, though 
brittle and apparently well petrified, is light buff or ivory in color, 
superficially resembling recent bone. 
The Curtis ranch locality is about 12 miles southeast of Benson, or 
about halfway between Benson and Tombstone in a straight line. 
The mesa at this point is cut back to form a large amphitheatre to the 
east of the San Pedro River. The locality where Gidley obtained 
the Curtis ranch rodent collection (pl. 48, fig. 1), one of the masto- 
donts, and a glyptodont (pl. 43, fig. 2) is about 2 miles in a northeast- 
erly direction from the Curtis ranch house, near the line between 
secs. 28 and 29, T. 18 S., R. 21 E., on land adjacent to the Curtis ranch 
proper. The rodent, lagomorph, and mustelid material collected by 
the 1936 party was all from Gidley’s locality, although that collected 
for the California Institute of Technology in 1928 was from a site 
about half a mile or more to the west and nearer the ranch house. The 
bone from the Curtis ranch embayment is light gray or nearly white 
to black, and often much checked, with calcareous material adhering 
to it. The small mammal jaws from the rodent locality are nearly 
all slate-gray or black in contrast to the buff or ivory colored Benson 
specimens. 
The manner in which the fossil materials accumulated in the San 
Pedro Valley deposits has been discussed by Gidley (1926, p. 84), and 
his rather vivid account is included herewith: 
The stratified beds of these localities consist principally of red clays, sands, 
and soft limestones that were evidently laid down in salt lakes of small extent 
in the central part of the Pliocene basin. 
The bones occur for the most part in relatively small patches or layers of 
greenish tuffaceous clay, which, according to Byran, interfinger on one side with 
arkosic gravel and conglomerate typical of deposition on alluvial slopes and on 
the other with the lake beds. This position seems to confirm Bryan’s view that 
these bone-bearing patches of greenish clay represent the marginal and fresh- 
water springs that are characteristic of the borders of salt lakes in such basins. 
The localities thus probably constituted the chief watering places for the animals 
of the region, and here naturally occur their fossil remains. 
That these areas were once boggy water holes is supported by the condition 
and arrangement of the bones they contain. For example, the skull of one of 
the mastodons was found completely covered by the undisturbed original matrix 
