ROCK    SPRINGS    COAL  FIELD,    WYOMING.  257 
The  field  sheets,  on  a  scale  of  2  inches  to  the  mile,  contour  interval 
50  feet,  are  now  being  compiled  into  a  base  map  on  a  scale  of  1  inch  to 
the  mile.  This  map  will  be  used  in  the  preparation  of  the  geologic 
and  other  maps  of  the  final  report.  The  sketch  map  (PL  XIV) 
accompanying  this  report  has  been  prepared  from  the  field  sheets  to 
show  the  more  prominent  cultural,  drainage,  and  economic  features, 
and  although  approximately  correct,  it  may  be  found  to  differ  in  minor 
particulars  from  the  final  large-scale  map. 
TOPOGRAPHIC   FEATURES. 
The  field  here  considered  lies  along  the  eastern  margin  of  the  great 
Green  River  Basin  and  includes  in  part,  on  the  east,  the  Great  Divide 
Basin.  It  lies  on  the  north  flank  of  the  Rock  Springs  dome,  which  is 
completely  surrounded  by  Tertiary  beds  and  does  not  form  a  topo- 
graphic basin.  Within  this  field  there  are  six  distinct  topographic 
districts,  each  presenting  an  entirely  different  character  and  texture 
from  those  of  its  neighbor.  Only  the  most  prominent  peaks  and 
ridges  that  encircle  the  dome  rise  above  the  adjacent  Tertiary  escarp- 
ments. The  divide  between  the  Green  River  and  Great  Divide 
basins  has  no  topographic  identity  or  distinctness.  It  lies  for  the 
most  part  several  miles  east  of  the  Rock  Springs  dome  and  is  formed 
in  some  places  by  Tertiary  rock,  in  others  by  Cretaceous  rock,  and  in 
still  others  by  igneous  rock.  The  divide  is  irregular,  extending  across 
several  distinct  topographic  districts. 
Within  the  field  the  principal  topographic  features  are  due  to  (1) 
hard,  resistant  sandstone  of  the  Mesaverde;  (2)  hard,  resistant  lime- 
stone and  sandstones  of  the  nearly  horizontal  Green  River  and 
Wasatch  formations;  (3)  migrating  sands;  (4)  soft  beds  of  the 
Wasatch,  and  (5)  igneous  rock. 
The  hard,  resistant  sandstone  of  the  Mesaverde  makes  notably  con- 
centric ranges  of  hills  more  or  less  continuous  about  the  central  por- 
tion of  the  Rock  Springs  dome.  These  concentric  ridges  contain  the 
most  important  coal  beds  of  the  field  and  are  usually  separated  from 
each  other  and  from  areas  of  equal  or  more  elevated  younger 
rocks  by  belts  of  low  relief.  The  two  most  pronounced  depressions 
are  carved  in  the  soft  shale  overlying  and  underlying  the  Mesaverde 
sandstone.  These  low  valleys  are  natural  routes  of  travel  and  afford 
easy  access  from  the  railroad  to  the  more  rugged  ridges  containing  the 
coal.  The  shale  below  the  Mesaverde  gives  rise  to  low  relief  along  the 
central  part  of  Rock  Springs  dome. 
The  hard,  resistant  limestones  and  sandstones  of  the  Green  River 
and  Wasatch  formations  produce  notable  table-like  forms,  bounded 
by  prominent  escarpments  of  considerable  length.  In  places  the 
harder  layers  in  these  beds  produce  bench  after  bench  in  regular  suc- 
cession, separated  by  nearly  parallel  valleys.     The  more  prominent 
71497— Bull.  341—00 17 
