Why crustal movements occur is not definitely known though many theories have been advanced to explain them. It is significant, nevertheless, that such movements have gone on in all parts of the world many times in history, and that in many regions they are taking place today. Parts of the California coast are known to be rising, while some other parts of the country are sinking slowly.
The question of whether or not the Grand Canyon region is still rising today cannot be answered with assuredness, but certain it is that it has been both above and below sea level at many different times in history.
It is the slow sinking of the region which has made possible great accumulations of sediments and the building up of rock layers. It is the rising of the land which has brought about great cycles of erosion such as apparently have occurred several times in the history of the Southwest. Since today this region is high above sea level, erosion is rapid. Not only does this elevation give the Colorado River and its tributaries speed and power to erode downward, but also it gives them unusual depth through which to cut and to carve the canyons which are their valleys. Without the rising of this plateau region, the colossal gorges, the weird, sculptured forms of erosion, all of the fascinating and picturesque features of this region could not be.
CUTTING THE GRAND CANYON
(PLIOCENE AND PLEISTOCENE EPOCHS)
“WHAT FORCE HAS FORMED THIS MASTERPIECE OF AWE?
WHAT HANDS HAVE WROUGHT THESE WONDERS IN THE WASTE?
O RIVER, GLEAMING IN THE NARROW RIFT
OF GLOOM THAT CLEAVES THE VALLEY’S NETHER DEEP,
FIERCE COLORADO, PRISONED BY THY TOIL,
AND BLINDLY TOILING STILL TO REACH THE SEA.