340 ANNUAL REPOET SMITHSONIAN INSTITUTION, 19 31 



EROSION AND SEDIMENTATION 



There is one process continuously active which is so simple that 

 apparently its influence on surface phenomena has been ignored or 

 even overlooked except by a few. This is the phenomenon of ero- 

 sion. Vast quantities of water fall to the earth each year and pre- 

 sumbably this has been going on continuously since the beginning 

 of the sedimentary age of the earth, the one that we are now in. 

 According to the best geological and geophysical evidence, the earli- 

 est sedimentary rocks were formed about a billion and a half years 

 ago. It is absolutely impossible for sedimentary rocks to be formed 

 without running water, and to have running water there must be 

 sloping ground. A succession of sedimentary rocks has been formed 

 during the past billion and a half years and for hundreds of millions 

 of years there have been living creatures on the earth, so it seems 

 perfectly logical to assume that rainfall must have occurred during 

 all of that period. 



The average rainfall per year over the land surface of the earth is 

 about 30 inches. Of course, there are regions where the rainfall is 

 100 inches or more, but these areas are very restricted in size, and 

 there are other areas, such as the great deserts, where there is no 

 rain at all or only a very few inches. A rainfall of 30 inches a year 

 amounts to about 1 mile in every 2,000 years, and during the whole 

 of the sedimentary age about 750,000 miles of rain could have fallen. 

 This, of course, means that by evaporation and precipitation the 

 ocean waters have been used over and over again. As the water of 

 the ocean is evaporated, the mineral content remains in the ocean. 

 When the water runs from the continental or island areas into the 

 oceans it carries in suspension or solution some solid material. The 

 solids are mostly in the form of salts. The mineral content of the 

 ocean waters that we now observe has been caused by this process of 

 evaporation and precipitation throughout the sedimentary age. 



This transfer of water from the oceans to the continents and then 

 back into the oceans would be of no consequence from a geological 

 standpoint if it were not for the resulting erosion of the exposed 

 surface of the earth. Much of the water runs directly to streams 

 and rivers and eventually reaches tidal water, except in a few desert 

 basins where the rivers have no outlet to the sea, but these latter are 

 very unimportant. The ^ater that runs to the sea carries much 

 material in suspension. The earth's surface is undergoing disintegra- 

 tion as the result of frost and chemical action. As soon as a particle 

 is loosened from a rock, it is subject to transportation to some other 

 place by wind or water. The effect of water in transporting material 

 is believed to be far greater than that of wind. In any cA-ent 

 tremendous amounts of material in suspension are carried by water 



