PLEISTOCENE HISTORY OF SAN JUAN MOUNTAINS 4o1 
Another large remnant of the San Juan drift occurs on a ridge 
1,000 feet above the Piedra and on the south side of that valley. 
Many of the bowlders in this deposit range from 5 to Io feet in 
diameter and a few are known which reach 25 feet in diameter. 
As the dissection of the country has progressed since the San 
Juan glacial epoch these bowlders have rolled down the valley 
slopes. 
From the distribution of these deposits the San Juan ice in this 
portion of the range must have been a large piedmont glacier which 
extended at least six miles beyond the mountains and had an areal 
extent on the lowlands of at least 30 square miles. The ice of 
Middle Fork, the Piedra, and several smaller canyons appears to 
have united to form this ancient glacier. The relief in this region 
during the San Juan epoch must have been much less than that of 
today. 
The complete physiographic map of this region has not been 
completed because of the need of a good topographic base, but it 
appears from an examination of the region that the San Juan 
glacial deposits are above and older than the bowlder mesa horizon, 
which it is believed has been correctly determined in this outlying 
country. 
The oldest drift yet found in the valley of the Rio Grande occurs 
on the north side of the valley high above the stream, at points 
between the moraines of the Bighorn epoch and the city of Creede. 
The fact that these deposits are high above the present valley and 
not on the intermediate slopes appears to have special significance. 
If the modern valley had been excavated at the time the San Juan 
ice there would surely have been remnants of the drift left some- 
where on the lower slopes in protected spots. Just east of the San 
Cristobal quadrangle this ancient glacial drift is found nearly 2,000 
feet above the present valley. Near Creede the deposit is repre- 
sented by a scattering of bowlders on the ridge just west of the city. 
These bowlders have probably been reworked and let down some- 
what from the position where they were left by the ice. This older 
drift is not difficult to recognize in the Rio Grande Valley because 
of the presence in it of crystalline rocks which must have come from 
the western margin of the San Cristobal quadrangle or from still 
