GHEAT   DIVIDE    BASIN    COAL   FIELD,    WYOMING.  223 
Lost  Creek  flats,  in  Tps.  23  and  24  N.,  Rs.  94  and  95  W.;  and  Red 
Desert  flats,  which  extend  northwestward  from  the  Union  Pacific 
Railroad  at  Red  Desert  through  Tps.  20  to  25  X.  and  Rs.  95  to  97  W. 
Most  of  the  large  and  many  of  the  small  depressions  are  occupied  by 
small,  shallow  alkaline  lakes  into  which  there  is  no  distinct  drainage. 
The  lakes  lie  at  or  just  below  the  mean  ground-water  level,  and  are 
supplied  by  ground  water,  being  much  larger  during  wet  than  during 
dry  seasons. 
All  of  the  remaining  area — 300  square  miles — lies  on  the  Atlantic 
side  of  the  watershed.  About  36  square  miles  in  Tps.  21  and  22  X., 
R.  88  W.,  near  Rawlins,  drains  eastward  into  North  Platte  River. 
The  remainder,  in  T.  26  X.,  Rs.  88  and  89  W.,  and  Tps.  27  and  28  N., 
Rs.  88  to  92  W.,  drains  into  Sweetwater  River.  The  only  permanent 
streams  in  the  area  are  situated  in  the  latter  region. 
GEOLOGY. 
STRATIGRAPHY. 
The  coal-bearing  rocks  of  the  area  here  described  are  of  Upper 
Cretaceous  and  Tertiary  ages.  They  consist  of  the  Colorado,  Mon- 
tana, Laramie,  undifferentiated  Tertiary,  and  Wasatch  formations. 
Of  these,  only  the  last  four  contain  workable  coal  beds.  Thin  beds 
of  coal  occur  in  the  basal  portion  of  the  Colorado,  but  at  no  place 
within  the  area  treated  do  they  reach  minable  thickness.  The  coal- 
bearing  formations  show  considerable  change  in  character  from  the 
southern  to  the  northern  edge  of  the  held.  Those  of  the  Cretaceous 
system  thin  rapidly  toward  the  north  and  the  outcrops  of  the  beds 
of  the  undifferentiated  Tertiary  become  more  and  more  narrow, 
owing  to  the  overlap  of  Wasatch  conglomerate.  The  accompanying 
section  (pp.  224-'_)25)  shows  the  thickness  and  general  characteristics 
of  the  coal-bearing  formations  near  the  Union  Pacific  Railroad  and  in 
the  gap  between  Whisky  Peak  and  the  Ferris  Mountains. 
