IOWA ACADEMY OF SCIENCES. 
255 
finally isolated altogether. Still beyond, the plain alone 
persists without notable mountains. This condition con- 
tinues on the one hand to the Gulf of California and on the 
other to the Gulf of Mexico. 
At the beginning of Tertiary time the region between 
the two great gulfs north to the present Colorado line must 
have been a vast lowland plain, with but faint relief feat- 
ures. A large part of this plain was on the bevelled edges 
of Cretaceous and older strata as is shown now in its rem- 
nants still clearly discernible. The Las Yegas plateau, 
the Llano Estacado, the bolson plains of central New Mex- 
ico and some of the less broken plains of eastern Arizona 
seem to belong genetically together. To the east and west 
of the vast area thus outlined a broad submarine platform 
was formed from the sediments derived from the planing 
off of the central land area. When the general bowing up 
of the region took place later in Tertiary times the great 
plain formed was partly a peneplane of destructional land 
origin and partly a constructional plain of marine origin. 
After the period of the main uprising, after the whole 
surface of the country had attained somewhat more than 
its present elevation above sea-level, normal faulting on a 
vast scale gave rise to numerous monoclinal block moun- 
tains, with a trend of north and south. There were num- 
erous halts in general movement and the Mesozoic and 
youngest Paleozoic beds here are completely stripped off 
the mountain summits. Several times the staying process 
has enabled partial peneplantation to take place. But the 
mountain blocks have become more and more tilted. 
Between Tertiary time and the present, enormous ero- 
sion has taken place. The vast plain has been deeply dis- 
sected by such old mountain-born streams as the Cana- 
dian, Pecos, Bio Grande, and Colorado. The valleys of 
these water courses are very wide and deep. On the east 
the Canadian flows 4,000 feet below the level of the old 
plain. The Pecos perhaps 2,500 feet. The Bio Grande 
about 1,500 feet. While the Colorado canyon is a mile 
in depth. 
