THE GEOLOGICAL BACKGROUND OF EVOLUTION 



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or sandy sea floors, where lived marine animals whose shells and bones 

 are imbedded in the sediments, now hardened into rock. In portions of 

 the seas distant from the ancient shores the water was clear, and there 

 deposits consisting almost entirely of the limy shells and hard parts of 

 organisms were formed and later consolidated into limestone. 



In many regions the sedimentary layers, or strata, are still horizontal, 

 as they were when laid down. But where they are piled up in the deepest 

 accumulations (sometimes several miles thick), they are commonly 

 folded and crumpled into great mountain ranges. This is the result of 



Fig. 25.1. Horizontal rock strata in the San Juan River canyon, southeastern Utah. The 

 canyon wall is more than 2,000 feet high. The lower strata are upper Carboniferous lime- 

 stones, shales, and sandstones; the higher ones are Permian sandstones, limestones, and 

 mudstones. (Photo by Henry P. Zuidema.) 



a sequence of events described below, which has been many times repeated 

 in earth history. 



In all the ancient continents there were some areas that remained 

 land most of the time. The rivers flowing off from these lands deposited 

 the coarser and bulkier sediments only a little distance offshore. Under 

 the weight of this accumulating sediment the crust sagged into long 

 narrow troughs bordering the ancient permanent lands, and these troughs 

 were filled with sediment as fast as they deepened. At various periods in 

 earth history there has been intense and long-continued compression 

 of the crust. At such times the weakest parts of the crust yielded, and 

 the long, narrow troughs filled with water-soaked sediments were weaker 

 than the masses of crystalline rock on each side. 



