June. 1937 



EVOLUTION 



Page Seven 



An Evolutionary Time Scale 



By A. M. WOODBURV 



Professor of Biology. University of Utah 



THE ordinarv visitor to Zion and Brvce Canyons of 

 southern Ltah and to the Grand Canyon of Arizona 

 little realizes the wealth of scientific information to be found 

 in the geological historv of the region and its significance in 

 the interpretation of evolution. The time scale in evolu- 

 tion depends for its interpretation largely upon the relative 

 positions of the different rock layers of the earth. Here, 

 the sedimentary rock layers, spread out in horizontal 

 fashion one above the other, produce a total depth of 

 about three miles. These layers have been relatively but 

 little disturbed so that they clearly show their relationships 

 to one another. 



The edges of these rock layers are so exposed to view 

 that they can be traced readily. The bottom-most mile 

 of these rock layers is exposed in the Grand Canyon 

 (Evolution Vol. 3, No. 4. p. 10, May, 1932). The second 

 mile of such layers may be found exposed between the 

 brink of the Grand Canyon and the top of Zion Canyon. 

 The third mile extends up the face of the higher plateaus 

 to the top of Bryce Canyon (See Chart). 



The extremely old rocks exposed in the bottom of 

 the Grand Canyon are the foundation upon which the 

 horizontal stratified sedimentary layers rest. It is obvious 

 that the lower rocks must have been in position before the 

 succeeding lasers could have been deposited on top. These 

 sedimentar>- layers deposited through the action of wind 

 or water thus show a successive series in time, the older 

 layers below and the younger ones above. This does not 

 imply it to be a continuous series. A layer once deposited, 

 may have been exposed to erosion and part of it worn 

 away before the next layer covered it. Such lapses of 

 time are referred to as unconformities and there are at 

 least ten major and many minor ones in these three miles 

 of rocks. Some of them represent lapses of time long 



enough for whole mountain ranges to be worn down and 

 planed off bv erosion. 



The Permian rocks (Kaibab limestone) at the top of 

 the Grand Canyon are covered by the Triassic rocks 3500 

 feet thick in the region below Zion Canyon. These are 

 covered b\ the Jurassic rocks 3050 feet thick in the Zion 



Courtesy United States Department of the Interior 

 BBYCE CANYON 

 Compare with Zion Canyon, pictured on our front page 



Region. These rocks are in turn buried by those of the 

 Upper Cretaceous, 2000 to 3000 feet thick, which reach up 

 to the bottom of Bryce Canyon. The topmost layer, 

 which is of Eocene age and buries all the others, is a 

 non-marine water-deposited limestone about 1300 feet 

 thick known as the Pink Cliffs. This is the layer in which 

 the indescribably esthetic carvings of 

 Bryce Canyon are cut. 



The fossils of algae, primitive as 

 well as higher invertebrates, fishes and 

 the fossil footprints of amphibians 

 found in the Grand Canyon are older 

 and more primitive than fossils found 

 higher up in the series. Fossil remains 

 or footprints are found in the layers 

 of the late Triassic or Jurassic rocks. 

 The writer has taken fossil petrified 

 trees from the earl\' Triassic. ganoid 

 fishes from the later Triassic and has 

 seen many reptile tracks in rocks of 

 the same age. Near Kanab in southern 

 Utah, on top of a projecting ledge 

 where the softer shale from above had 

 worn away, a series of dinosaur tracks 

 were exposed to view. They were as 

 plain and unmistakable as the cow's 



DI.\GR.\MM.\TIC PROFILE 



Columnar Section sliowing stratigraphic 



rela(ion.s of IJryce, Zion and 



Grand Canyons 



