8 FIELD OPERATIONS OF THE BUREAU OF SOILS, 1903. 



and pleasant. Owing to the frequent severe frosts occurring in the 

 valley during the winter season the growing of citrus fruits is impos- 

 sible. The average date of the last killing frost in spring at Fort 

 Grant is April 7; at Dudley ville, March 30; and of the first in fall, 

 Fort Grant, November 29; Dudley ville, November 11. 



During the fall and winter the wind movement is moderate. With 

 the advance of spring, however, the winds blow with increasing force 

 and severe wind and dust storms often occur during the spring and 

 summer. 



PHYSIOGRAPHY AND GEOLOGY. 



The mountains of this area consist of rugged chains and spurs, 

 often rising in perpendicular cliffs and capped with jagged peaks. 

 Onl} r the tops of the higher ranges are forested or covered with other 

 than desert vegetation. Between the ranges lie level valleys, usually 

 traversed by a stream, often of intermittent flow. Spreading outward 

 from the base of the mountains to the valleys below is a succession of 

 evenly sloping or gently undulating plains. These plains, formed of 

 mountain waste brought down by swiftly moving flood streams, head 

 in the narrow canyons of the mountain slopes, from which they extend 

 in broad, sj^mmetrical, cone or fanlike deltas. As they recede from 

 the canyons the areas coalesce, forming a single, broad debris apron 

 at the foot of the mountains. The valleys have in many cases been 

 filled to great depths by this material. 



The Gila Valley is almost wholly inclosed by rugged mountain 

 ranges. Upon the north and east it is bounded by the generally bar- 

 ren outlying peaks and hills of the Gila Range, through which the 

 Gila River cuts at the Narrows, some 10 miles above Solomonsville. 

 Along the southwestern boundary it is inclosed bj^ the Pinaleno Range, 

 culminating in the rugged and lofty peak of Mount Graham. This 

 peak rises to a height of over 10,000 feet above sea level, and until 

 late in spring retains the winter snows in its lofty canj^ons and on its 

 forest-covered slopes. To the northwest the valle} 7 is much con- 

 tracted by scattered peaks and ranges, the highest of which is Mount 

 Trumbull. 



The mountains consist primarily of granite, with frequent intrusions 

 of volcanic rocks, usually lavas of recent date. Upon the upper slopes 

 of the higher peaks springs and streams are abundant. 



Approaching the San Simon Valley, the southwestern side of the 

 Gila Valley is bounded by the undulating plains lying at the foot of 

 the Pinaleno and Peloncillo ranges. 



The debris apron extends nearly or quite to the valley trough, being- 

 interrupted only by recent erosion of the valley streams. The agency 

 of the canyon streams in building up this great sheet, composed 

 mostly of material derived from granitic and volcanic rocks, is appar- 

 ent. The soils are usually of a reddish or chocolate-brown color and 



