NATURE OF THE GEOLOGIC RECORD 131 



walls of the Grand Canyon of the Colorado River. The many strata visible 

 there afford us information concerning successive changes in that region of 

 the earth for many millions of years. 



How, then, do we tell which is "page 1 ," which "page 2," and so on in our 

 geologic record "book"? As a general principle we may state that the oldest 

 strata are the deepest ones, and that as we proceed upward in such a series 

 of layers as that displayed in the walls of the Grand Canyon the strata are 

 successively younger and younger in age. TTiis time sequence follows natu- 

 rally from the manner in which the material is deposited, as just described. 



Disturbance of the Record 



Interpretation of the sequences of events in the geologic record would be 

 relatively simple if confusing and destructive forces were not at work. One 

 destructive force is erosion, which removes many "pages," entire "chap- 

 ters" even, from the record. For hundreds of thousands or millions of 

 years a certain region is covered by shallow sea and receives successive de- 

 posits. If the earth's crust is sinking slowly, as portions of the Atlantic 

 coastline of North America are known to be doing today, the deposits may 

 eventually total thousands of feet in thickness. Finally we may picture a 

 great upheaval occurring in that region, with formation of a mountain 

 range. In the process the deposits formed in the bottom of the sea are thrust 

 up into the air, thousands of feet up perhaps. This sort of thing has hap- 

 pened time and again in the history of the earth. Indeed, if it had not hap- 

 pened we should know relatively little of the past history of the earth, since 

 obviously deposits are almost inaccessible to human study as long as they 

 are buried under the sea. Yet as soon as the strata are exposed to the air 

 in this fashion the forces of erosion start to tear them down, and the rivers 

 to transport the products of their destruction to the neighboring sea, 

 thereby initiating the formation of new deposits in that sea. Thus the sub- 

 stances composing the earth's crust are being constantly "reworked" — 

 portions exposed to the air being eroded away, the products of this erosion 

 going into the formation of new deposits which will eventually form new 

 sedimentary rocks. 



As erosion continues on the newly formed mountain range entire strata 

 may be worn away, and eventually entire groups of strata. This process is 

 going on in our present mountain ranges; since it is slow geologists have 

 ample opportunity to study the strata before they disappear. But consider 

 all the strata which were destroyed before there were any geologists — or 

 any men at all, for that matter. We are told that at one time the Appalach- 



