500 Bulletin 84 



the Grand Canyon of the Colorado. The peculiar erosion at this 

 time was probably due to both a gradual uplift in the vicinity of the 

 present canyons and downfaulting of neighboring regions. Three 

 distinct periods of volcanic activity are evident, the last being in 

 some cases very recent. 



SOILS 



Many strata outcrop in belts on the surface of the plateaus, 

 sandstones and shaly sandstones covering the largest area. Soils 

 resulting from the weathering of these strata as a rule are rich in 

 iron, poor in clay, and infertile. In some places sandstone strata 

 are covered with a thin veneer of limestone, and the resulting soil 

 is somewhat more productive. Strata of lava cover the next largest 

 area. Soil formed by decomposition of this material is usually high 

 in fertility, but in the Colorado Plateaus region most of the lava 

 has appeared too recently to allow of much more than mechanical 

 disintegration. Limestone strata outcrop on quite extensive areas, 

 the principal one being the Kaibab Plateau, most of which lies north 

 of the Grand Canyon. Well weathered limestone soils are very fer- 

 tile. A considerable area of alluvial soils is found. The abrupt 

 slope of the Colorado Plateaus Province, averaging about 200 feet 

 per mile, assists rapid erosion by the young rivers of the region. 

 The finer and more disintegrated portions of exposed strata are 

 transported great distances, separated by gravity, and deposited 

 in the broader valley bottoms. Soils thus formed, composed of the 

 finest materials of various strata, are very fertile. No general state- 

 ments apply to mountainous parts of the district since they are 

 regions of complex and profound faulting. 



WATER SUPPLY 



The general arrangement of strata in the plateaus is vmfavor- 

 able for economic development of groundwater. In places wells 

 have been drilled to a depth of more than 1500 feet without success. 

 However, water is occasionally found near the surface of the pla- 

 teaus, but only in localized spots, and in many cases it is so charged 

 with minerals that it is unfit for domestic use. Water may usually 

 be developed at a slight depth along the main courses of the valleys, 

 but it is often strongly alkaline. In some places, notably near St. 

 Joseph, artesian water has been found at depths of 200 to 800 feet. 



