1920] PALMER, CANYON FLORA OF EDWARDS PLATEAU 233 



THE CANYON 



PLATE 



OF TEXiVS 



Ernest J. Palmer 



Any one who has traveled by daylight over the Southern Pacific railway 

 from San Antonio towards El Paso can scarcely have failed to notice the 

 bold line of hills and cliffs rising abruptly from the plain and visible on the 

 northern sky-line for nearly one hundred miles, and to have contrasted it 

 with the comparatively level country through which he was passing and 

 which extends as far as can be seen to tlie southward. This escarpment is 

 part of a long line of faulting traceable from the vicinity of the Red River in 

 northern Texas to near the mouth of the Pecos; along this line in remote geo- 

 logical times a great fracture of the earth's crust occurred w^hich threw down 

 the region to the south some hundreds of feet below its former level and 



allow^ed the sea again to invade it. 



The Rio Grande Plain, thus set off from the highlands, has a slightly un- 

 dulating surface diversified here and there by low, rocky hills and ridges 

 and traversed by numerous shallow% gravel-covered stream beds which, ex- 

 cept for infrequent water holes, are dry through the greater part of the year. 

 A thin gro^\'th of stunted Mcsquite, Chaparral, Opuntias and other semi- 

 desert shrubs covers much of the area, amongst which after the infrequent 

 rains a growth of short grass and herbage springs up and affords a precari- 

 ous pasturage to the herds of cattle that have succeeded the buffalo, deer 

 and antelope that formerly roamed over it. 



Above tlie escarpment is the Edwards Plateau, extending west to the 

 Pecos River and indefinitely northw^ard until merged into the Staked Plains, 

 The plateau Is underlaid by nearly horizontal beds of limestone aggregating 

 many hundreds of feet in thickness, the different layers varying greatly in 

 hardness, texture and purity. The old Cretaceous seas in which these 

 strata were deposited must have fairly teemed with animal life, as is evi- 

 denced by the beautifully preserved sea urchins, oyster shells and other fos- 

 sils found in countless numbers in some of the beds. The erosion of ages 

 working upon these more or less solvent limestones has carved out a topog- 

 raphy for the region that is varied and striking and in places picturesque 



and fantastic. 



The average elevation of most of the plateau ranges from about one thou- 

 sand feet on the south and east to three thousand or more in the interior and 

 northwestern portion. The rainfall is rather scant, the annual average di- 

 minishing from twenty-five or thirty inches on the eastern side to less than 

 twenty on the western. A considerable proportion of it, however, is 

 absorbed by the porous limestones that underlie much of the area. 

 Through 'Ihis " honeycomb rock '' the water percolates until it strikes an 

 impermeable layer over which it flow^s until it finds escape in some of the 

 innumerable fissures or canyons that dissect the marginal rim of the high- 

 lands. Most of the precipitation occurs in the autumn and winter months, 

 and the rains are often of a torrential nature, so that sometimes from a 



