42 IOWA ACADEMY OF SCIENCES. 



ing animals, in that segregation of bowlders in the upper 

 part of the drift and immediately beneath the fine surface 

 soil, which is so noticable in northern and northwestern 

 Iowa, especially within the Wisconsin drift lobe (Plate II). 



From the very nature of their distribution and habits 

 living plants affect chiefly the last, most recent, less com- 

 pacted superficial deposits of the earth. Their influence 

 is usually exerted so slowly that the immediate results are 

 not striking, and never so pronounced as those sometimes 

 produced by water. Nevertheless they are sufficient to 

 warrant us in placing living plants among the most effec- 

 tive agencies concerned in the formation and transforma- 

 tion surface deposits of the earth. 



It is commonly held that plants destroy the rock with 

 which their roots come in contact, yet one of the most 

 striking effects of their presence is the prevention of 

 erosion. Friable rocks are often held together against the 

 erosive action of air and water by surface coatings of 

 lichens and liverworts. Upon the sandstone bluffs at 

 Wild-cat Den, in Muscatine county, the lichen Lecidea albo- 

 ccerulescens (Plate III), forms incrustations which finally 

 stand out in relief as the surrounding sandstone is worn 

 away, and similarly liverworts, such as Conocephaliis coni- 

 cus and Asterella liemispliterica Beauv. (Plate IV), bind 

 the grains of sandstones together by their rhizoids, and 

 prevent erosion. Even much harder and more resisting 

 rocks are often protected by plants. Thus lichens, such as 

 Binodina ore'uia (Ach.) Mass. (Plate V), and others, form 

 incrustations upon exposures of the Sioux Quartzite in 

 northwestern Iowa, and protect the rock from wind-driven 

 sands in much the same manner that wax or paraffin pro- 

 tect glass surfaces from the sand-blast. The hard quartz- 

 ite is usually highly polished upon the bare surfaces, while 

 lichen-covered areas, equally exposed, protect the rock 

 against the natural sand-blast to which they are so often 

 exposed. 



But this preventive power is more strikingly shown, as 

 a rule, in the case of looser materials, such as loess and 

 drift clays, residuary clays, soils, etc. If a surface of such 



