SHAPING THE EARTH — BOWIE 343 



the process of erosion and sedimentation, the isostatic balance is dis- 

 turbed. It is then that gravity comes into play and causes the sub- 

 crustal material to move horizontally to restore the balance. We 

 have evidence to show that as much as 6 or 8 miles of sediments 

 have been deposited in the areas along the shores of an inland sea 

 or the margin of an ocean. This load of material pushed the crust 

 down into hotter regions. Each particle of crustal material reached 

 a position several miles below the one it formerly occupied. The 

 geoisotherms were depressed with the crustal material. Eventually, 

 probably millions of years after the cessation of the sedimentation, 

 the geoisotherms returned to their normal positions. In doing so, 

 the crustal material which had been depressed increased in temper- 

 ature, perhaps as much as 400° C. in extreme cases. 



This increase in temperature, of course, expanded all of the crust 

 below the sediments. The expansion tended to be cubical, that is, in 

 all directions, but the material involved was restrained from move- 

 ment except in the upward direction ; hence, the result of the expan- 

 sion was an uplift of the earth's surface. The amount of move- 

 ment could not have been sufficient to form great mountain masses 

 rising 2 or more miles high, but is it not possible that certain chem- 

 ical or physical changes, other than normal expansion took place in 

 the crustal material and that this independent expansion gave the 

 added height to the uplifted surface? This idea is in complete har- 

 mony with isostasy and I believe it has much merit. 



When an area is undergoing erosion, it is not lowered at a rate 

 comparable with the rate of erosion. If a thousand feet of material 

 is eroded from a mountain area, the crust below will move upward 

 by the influx of subcrustal material which restores the equilibrium. 

 The crust will presumably rise up 800 or 900 feet as a result of the 

 1,000 feet of material taken from the surface. If a mountain area 

 has an average elevation of about 2 miles, from 5 to 10 miles of 

 material, or even more, will have to be eroded away, if erosion is 

 the only acting agent, before the area is brought to a low level where 

 erosion practically ceases. During this process every cubic yard of 

 material in the crust below the erosion area will have been brought 

 upward 5 or 10 miles or more into colder regions. Eventually the 

 geoisotherms, which have been deflected upward, will resume their 

 normal positions, and in consequence each particle of the uplifted 

 crust will become colder by several hundred degrees centigrade. 

 This causes contraction and the surface becomes depressed. The 

 surface may be depressed even below sea level, in which case new 

 material in the form of sediments will be deposited in the trough 

 or basin that is formed. There is evidence that mountain areas 

 have been elevated and depressed several times and the explanation 



