22 INDIANS OF THE SOUTHWEST. 
DISTRIBUTION. 
As might be expected in a semi-arid region, the agri- 
cultural population in prehistoric times was concen- 
trated at the higher elevations where the rainfall was 
the greatest and in the river valleys where irrigation 
could be easily practised. 
San Juan. One of the most important regions 
anciently occupied was that watered by the northern 
tributaries of the San Juan River. These streams are 
fed by the snows of the mountains of southern Colorado 
and Utah. At some distance from their sources they 
are confined in sheer-walled canyons which unite with 
each other as they approach the San Juan, which enters 
the Colorado above the Grand Canyon. Some of the 
ruins are on the tablelands between the streams, others 
are at the heads of the canyons, and many are in the 
canyons themselves either on their floors or under their 
overhanging walls. Cliff Palace and Spruce Tree House 
are two of the largest and best known ruins standing 
under cliffs. In the valley of the Animas is a very large 
ruin in the open valley near the town of Aztec. 
In Chaco Canyon, there is a cluster of eleven large 
ruins which evidently represent an important political 
group of prehistoric villages. One of these, Pueblo 
Bonito, is hardly surpassed in size and interest anywhere. 
Canyon de Chelly, which joins Chinlee Valley, has 
many ruins both on the floor of the valley and under 
the walls. 
Rio Grande. On the western side of the Rio Grande 
Valley are many large ruins. Some of them are in the 
valley of the Rio Chama; many of them are on the 
mesas of the Pajarito tableland south of it; and others 
are in the canyon of the Rito de los Frijoles. In the 
valley of the Rio Grande itself and along its eastern 
