258         CONTRIBUTIONS    TO   ECONOMIC    GEOLOGY, 
ridges  form  in  many  places  precipitous  bluffs,  while  in  parts  of  the 
field  the  beds  present  characteristic  badland  topography. 
Huge  sand  dunes  formed  by  the  migrating  sands  are  the  most  con- 
spicuous topographic  feature  in  the  northern  part  of  the  field. 
The  soft  beds  of  the  Wasatch  form  a  low  depression  along  the  syn- 
clinal trough  of  the  Great  Divide  Basin.  This  formation  contains  num- 
erous beds  of  coal,  few  of  which  are  well  exposed.  The  topographic 
features  are  such  that  the  coals  are  readily  accessible. 
In  this  field  igneous  rocks  rise  like  landmarks  out  of  the  arid  pla- 
teaus of  the  Red  Desert.  The  lava  forming  these  rocks  flowed  from  a 
little  group  of  volcanoes  far  removed  from  similar  centers  of  igneous 
activity.  Their  nearly  concentric  cones,  needle-like  necks,  irregular 
dikes,  and  table-like  sheets  afford  a  striking  contrast  to  the  topo- 
graphic features  of  the  surrounding  hills. 
The  drainage  for  the  most  part  flows  to  the  Pacific  coast.  Part  of 
the  area,  however,  on  the  northeast  lies  within  the  Great  Divide  Basin, 
which  has  no  outlet.  The  main  drainage  is  not  affected  by  the 
Rock  Springs  dome,  which  in  a  way  connects  the  two  mountain  ranges, 
Uinta  and  Wind  River.  Bitter  Creek,  the  only  prominent  stream  in 
this  field,  flows  across  the  central  portion  of  the  dome  at  nearly  right 
angles  to  the  major  axis  and  has  carved  a  broad  valley,  along  which 
the  Union  Pacific  Railroad  was  constructed.  The  three  large  tribu- 
taries of  Bitter  Creek,  however,  have  been  shaped  indirectly  by  the 
fold.  On  account  of  the  difference  in  the  hardness  of  the  beds  these 
valleys  extend  along  the  strike  of  the  beds  and  are  approximately  at 
right  angles  to  Bitter  Creek.  All  the  area  not  tributary  to  Bitter 
Creek  drains  to  the  east,  and  the  water  flows  into  the  Great  Divide 
Basin,  which  includes  an  area  of  approximately  4,200  square  miles. 
The  divide  between  these  two  drainage  systems  lies  along  the  crest  of 
Cathedral  Bluff  or  Laney  Rim,  swings  northwestward  and  crosses  the 
Union  Pacific  Railroad  track  just  west  of  Robinson,  thence  trends  in 
a  northwesterly  direction  to  Steamboat  Mountain,  thence  northward 
to  South  Pass,  where  the  east  and  west  divides  reunite. 
The  location  of  this  divide  within  the  area  may  be  seen  on  the 
accompanying  map,  PI.  XIV,  also  on  PI.  XII. 
In  the  Great  Divide  Basin  there  are  numerous  small  dry  lake  beds, 
alkali  lakes,  and  alkali,  clay,  and  red  flats,  the  largest  being  the  Red 
Desert  flat,  near  the  center  of  the  basin.  In  the  northern  part  of  the 
field,  within  the  sand-dune  area,  there  are  also  numerous  small  lakes 
that  are  fed  by  melting  snow  and  ice  in  and  beneath  the  sand  dunes. 
Throughout  this  region  very  few  rocks  are  exposed,  and  the  dip 
slopes  are  well  covered  by  gravel  and  loose  soil. 
