10 H. E. Gregory — A Geologic Iieconnaissance 



The three basins. — The expanded portion of the valley floor 

 between Cuzco and the Angostura Narrows may be termed 

 the Cuzco Basin. It is an intensively cultivated area, the 

 seat of Inca civilization at the time of the Spanish conquest, 

 and the present home of over 90 per cent of the inhabitants of 

 the Cuzco Province. The basin is bordered by gravel fans 

 and low cliffs cut in poorly consolidated sediments. As viewed 

 from its eastern margin the floor of the basin is flat, but near 

 at hand it is seen to consist of two portions separated by a 

 low rock ridge thickly mantled with sand. This ridge has 

 little topographic importance but serves to separate two areas 

 of tillable land and to determine the location of the villages of 

 San Sebastian and San G-eronimo. The portion of the basin 

 floor west of this agricultural divide is almost without relief 

 (fig. 4). As seen from Ttica-Ttica the minor inequalities on 

 the floor of the basin disappear and a graded surface, sloping 

 gently eastward and broken only by groves of eucalyptus and 

 the houses of the city of Cuzco, is presented to view (fig. 5). 

 The landscape is a garden with plats of barley and potatoes, 

 bordered by high stone walls and traversed by irrigation ditches. 

 In the vicinity of San Geronimo the flat floor of the Cuzco 

 Basin is confined to a strip half a mile wide bordering the 

 Huatanay. The village itself is set on the edge of an enormous 

 alluvial fan built outward from the mouths of rock-walled 

 canyons which score the slopes of Picol (fig. 6). 



The topographic features of the. Cuzco Basin are duplicated 

 in several respects by the Oropesa Basin (fig. 7). The 

 Oropesa Basin has, however, less width and a flatter floor 

 than its larger companion. The Lucre Basin shows the least 

 relief of the three subdivisions of the Cuzco Valley. 



Drainage. — The sinuous line marking the rim of the Cuzco 

 Valley forms a rough rectangle with a length of 22 miles and 

 an average width of 7 miles. The 154 square miles thus 

 inclosed is drained by a single stream, the Huatanay, which 

 occupies an axial position from its source to the village of 

 Huacarpay. Thence it turns sharply northward, escapes 

 through the valley wall, and joins the Urubamba at Sierra-Bella. 

 The Huatanay rises in flat-floored glaciated swales at the base 

 of Cerro Seneca, where numerous springs issuing from till 

 insure perennial flow. During the rainy season the supply 

 obtained from ground water is supplemented by torrents 

 which traverse deep rock grooves, cut in the mountain slopes 

 to heights of 14,000 feet. Through the city of Cuzco the 



