SOUTHERN PACIFIC LINES 69 
The most notable physiographic features in the San Antonio region 
are the wide plains and terraces which have been developed by ero- 
sion and deposition by streams on the surface of the soft clays of the 
Tertiary and Cretaceous formations. The largest plain, which lies a 
short distance above the narrow alluvial strips bordering the streams, 
extends from the San Antonio River to Leon Creek and is occupied 
by Kelly Field and other aviation stations. It is covered in greater 
part by a sheet of gravel and loam, mostly from 10 to 20 feet thick, 
and has an elevation of 600 to 700 feet, with gentle slope to the 
south. A smaller but similar plain lies between Salado (sa-lah’do) 
and Rosillo (ro-see’yo) Creeks, east of the city. The highest plain, 
which occupies the ridge between the valleys of Salado Creek and the 
San Antonio River and is about 100 feet higher than the adjoining 
area, represents the Uvalde Plain referred to on page 62 (footnote). 
It is capped by a sheet of gravel and loam which is well exposed in 
the long cut of the Missouri-Kansas-Texas Railroad in the southeast- 
ern part of the city, as shown in Plate 8, B, and Fort Sam Houston is 
also built on its smooth surface. There are many outliers of this plain 
farther north and west. 
Not far north of San Antonio there are excellent exposures of 
slabby Eagle Ford limestone, Buda limestone, and Del Rio yellow 
clays with abundant Exogyra arietina, and in the hills of the Edwards 
Plateau, of Georgetown, Edwards, and Glen Rose limestones, which 
carry many distinctive fossils. 
Westward from San Antonio the railroad goes south for 1% miles 
and then, turning abruptly west, crosses the San Antonio River and 
the wide alluvial plain and skirts the east side of Kelly Field. This 
plain is wide and level because it is developed on the soft clays of 
the Upper Cretaceous. It is capped by a sheet of alluvial loam de- 
posited by Leon Creek and other streams in relatively recent geologic 
time. Leon Creek is crossed just beyond Leon siding, and the low 
rolling hills of the Midway formation capped by gravel of the higher 
terrace level are traversed between Leon Creek and Medio Creek. 
A mile west of Leon Creek the railroad bends around the south 
end of a ridge, showing ledges of buff sandstone of the Midway for- 
mation, which with low dip to the west passes under 
Macdona. the Indio formation at Medio Creek. All the lower 
ae ape lands from this point westward past Macdona and 
New Orleans 590 miles. Lacoste (turn to sheet 10) are underlain by alluvial 
sand and gravel deposited by the Medina River, 
which is crossed a mile east of Macdona. This stream rises in the 
51 In the bank of Leon Creek about | clay or Escondido (p. 71), a relation 
a mile north of the railroad is an excel- | which is revealed at intervals up the 
lent exposure of the unconformable con- | creek to the fault that crosses the 
tact of the Midway on Navarro sandy | st 4 miles above the railroad 
