richardson.]      SALT,    PETROLEUM,    ETC.,    IN    TRANS-PECOS    TEXAS.      575 
is  the  shallow  lake  which  is  near  the  center  of  the  basin,  and  is  about 
3|  miles  long  and  one-half  mile  wide.  Most  of  the  local  depressions 
are  marsh  areas  that  contain  water  only  for  a  short  time  during  the 
rainy  season. 
The  Sierra  Diablo,  marking  the  eastern  scarp  of  the  Diablo  Pla- 
teau, which  is  an  extensive  region  lying  beyond  the  area  here  con- 
sidered, delimits  the  Salt  basin  on  the  west.  These  mountains  extend 
about  25  miles  from  north  to  south,  and  are  flanked  on  the  south, 
east,  and  northeast  by  an  abrupt  escarpment,  which  is  steepest  on 
the  east,  where  it  rises  over  2,000  feet  above  the  Salt  basin.  The 
mountains  are  capped  by  massive  limestone  that  dips  westward  at  a 
low  angle,  with  which  the  general  slope  of  the  surface  corresponds. 
Very  little  of  the  drainage  passes  directly  eastward  into  the  Salt 
basin,  the  principal  outlets  being  to  the  south,  west,  and  northeast. 
At  the  eastern  base  of  the  Guadalupe-Delaware  Mountains  there  is 
a  gently  undulating,  east-sloping  plain  that  is  underlain  by  gypsum. 
Within  Texas  this  area  is  approximately  50  miles  in  length  and  aver- 
ages 15  miles  in  width,  though  at  the  State  boundary  it  is  25  miles 
wide.  The  general  flatness  is  relieved  by  occasional  isolated,  rounded 
hills  that  are  capped  by  limestone.  The  gypsum  plain  is  traversed 
by  Delaware  Creek  near  the  State  line  and  farther  south  by  Horseshoe 
and  Cottonwood  draws.  These  waterways  occupy  broad  valleys,  in 
which  narrow7  gorges  locally  have  been  intrenched. 
Eastward  a  narrow  range  of  low  hills  intervenes  between  the  gyp- 
sum plain  and  Pecos  Valley.  These  are  the  Screwbean-Rustler  hills, 
which  extend  southwestward  from  the  mouth  of  Delaware  Creek,  a 
distance  of  about  40  miles,  in  a  belt  averaging  5  miles  in  width. 
They  rise  3,600  to  3,700  feet  above  sea  level  and  150  to  250  feet  above 
the  adjacent  plains.  They  are  capped  by  limestone,  and  when  viewed 
from  an  elevation  show  an  even-topped  sky  line.  Erosion  has  dis- 
sected these  hills  somewhat,  and  Horseshoe  and  Cottonwood  draws 
flow  through  them  in  relatively  narrow  valleys. 
In  the  vicinity  of  Kent  and  San  Martine,  at  the  western  end  of  the 
Toyah  basin,  are  a  number  of  detached,  rounded  and  irregularly 
[(shaped  hills  that  rise  from  100  to  200  feet  above  the  surrounding  level. 
[These  hills  are  composed  of  low-lying  Cretaceous  rocks,  which  are 
khe  erosion  remnants  of  a  former  connected  mass. 
The  Toyah  basin  is  that  part  of  the  Pecos  Valley  lying  between  the 
vicinity  of  the  Texas-New  Mexico  boundary  and  the  escarpment  of 
the  Stockton  and  Edwards  plateaus,  50  to  70  miles  south  of  the  Texas 
and  Pacific  Railway,  the  east- west  limits  of  the  basin  being,  respec- 
tively, the  scarp  of  the  Staked  Plains  and  the  mountains  to  the  west. 
This  large  area  is  underlain  to  a  great  but  unknown  depth  by  uncon- 
solidated materials  which  may  be  in  part  lake  deposits. 
Bull.  260—05  m 37 
