154 



SOIL 



Cottonwood, Populus deltoides, have this same abiUty, to a consider- 

 able extent. A cotton wood tree may become two or more times its 

 normal height as the sand piles up around it but by putting out 

 adventitious roots into the sand the distance from the top of the tree 

 to the source of water supply is kept normal. In the gypsum sands 

 in White Sands National Monument, New Mexico, the cottonwood, 

 Populus wislezenii, and two shrubs, the shrubby pennyroyal, Polio- 

 mintha incana, and the squaw bush, Rhus trilobata, (F'ig. 67) have 

 similar habits. 



Another sort of instability of soil is that caused by water erosion. 

 This takes place along streams and on unprotected hillsides and 



Fig. 67.— Rhus trilobata (squaw bush), holding a mound of gypsum sand in 

 White Sands National Monument, New Mexico. The shrub grew upward as 

 fast as the sand piled up around it. Later the surrounding sand was blown 

 away leaving only the mound. 



often is very destructive to vegetation. When a hillside is covered 

 with forest practically no erosion is possible, but when such a forest 

 is removed gulleying often sets in. Large areas have in some places 

 been made useless for agricultural purposes by thoughtlessly re- 

 moving the forest from an adjacent hillside. In such a case erosion 

 starts on the hillside and rapidly cuts back into the level upland. 

 The "bad lands" of North and South Dakota and Nebraska, which 

 have been produced through erosion, are in some portions almost 

 destitute of plant life (Fig. 68). Wind and water erosion have be- 



