THE GREAT LAND CREATION<br > The forces that created the geology of the northern Arizona. Toward the end of the<br >Southwest have also conspired to lay it Mesozoic， North America broke away from<br >bare. The region lies in the latitude of the Europe， collided with the Pacific plate， and<br >world s great deserts， where air currents overrode it， initiating 25 million years of<br >thrown skyward by equatorial heat， drained mountain-building known as the Laramide<br >of their tropic rains， descend in dry， eva- Orogeny. The area was volcanically active，<br >porative winds. Moisture-laden Pacific and the Arizona Highlands and Rocky<br >storms slam into the Sierras and the Peninsu- Mountains derive from this time. Hot<br >lar ranges in California， rise， cool， drop their mineral-bearing magma seeped through the<br >rain and snow on the mountains， and con- fractures， depositing the copper now exten-<br >tinue dry over the same inland basins. The sively mined in Utah and Arizona.<br >resulting lack of vegetational cover acceler- Uplift continued into the Cenozoic， be-<br >ates erosion， gouging out history-revealing ginning 65 million years a~o， elevatin~ the<br >canyons， scarps and mesas. Sonthwesterfi Colorado Plateau--that immense oval of<br >weather pleases the geologist thoroughly， high land that encompasses most of southern<br > One should bear in mind that of the earth s Utah， northern Arizona， northwestern New<br >some 4V2 billion years of history， only the last Mexico and southwestern Colorado. The<br >half billion is documented in any detail. The Colorado River cut into the Plateau as it<br >first 89 percent of earthly time is known rose， giving us perfect core samples of the<br >collectively as the Precambrian， and the Plateau s interior， and revealing quite a<br >oldest Southwestern evidence of its events surprise: for all the collisions of plates，<br >are the dark metamorphic rocks of the volcanism and assorted calamities， the stra-<br >Grand Canyon s inner gorge. They began as ta of the Grand Canyon have reposed in<br > J: 4 : b，， Aon~，-;tocl "~ t-mlrn ht~ri z+lnt:~l~ which ~，eolo~~ists refer to as +<br >