GLENROCK   COAL  FIELD,   WYOMING.  153 
ted  along  North  Platte  River,  and  the  Chicago  and  Northwestern 
Railway  runs  through  these  towns,  mostly  in  the  bottom  land  of 
the  river. 
Throughout  a  considerable  part  of  the  field  the  building  of  first- 
class  roads  would  be  difficult  on  account  of  the  sandy  nature  of  the 
soil.  There  is  no  other  route  so  well  adapted  to  railroad  building  as 
the  river  valley,  but  it  would  be  possible  to  build  a  railroad  to  any 
point,  as  the  country  as  a  whole  is  not  rough,  and  cuts  and  fills  could 
be  made  without  the  excavation  of  much  rock.  The  main  obstruction 
to  a  north-south  railroad  is  the  divide  between  North  Platte  and 
Powder  rivers,  which,  however,  presents  no  great  difficulty,  as  it  is 
only  500  to  1,000  feet  higher  than  the  valley  of  the  North  Platte. 
GEOLOGY. 
STRUCTURE. 
The  structure  is  dominated  by  the  uplift  of  the  Rocky  Mountain 
front  range  on  the  south  and  a  slight  uplift  on  the  west.  The  strata 
dip,  in  general,  north  at  the  east  end  of  the  field  and  northeast  at  the 
west  end,  so  that  the  structure,  as  a  whole,  is  that  of  a  broad,  shallow 
syncline  pitching  to  the  northeast.  In  a  belt  6  or  8  miles  wide,  lying 
along  the  foot  of  the  mountains,  the  average  dip  is  15°,  but  it  varies 
greatly  in  this  belt,  as  does  also  the  strike.  Along  the  southern  border 
of  the  field  the  strata  in  this  belt  are  commonly  upturned  very  steeply, 
but  farther  from  the  mountains  the  strata  are  almost  horizontal. 
No  great  faults  affect  the  area  of  coal-bearing  rocks,  though  there  are 
probably  numerous  small  ones.  For  example,  there  is  one  in  the 
Glenrock  mine  which  has  a  throw  of  8  feet. 
STRATIGRAPHY. 
GENERAL    SECTION. 
The  following  formations  occur  in  the  Glenrock  field   (see  also 
columnar  section  on  map,  PI.  IX). 
Quaternary:  Wind-blown  sand,  rain  wash,  mesa  and  terrace  gravel,  lake  beds,  and 
flood-plain  deposits. 
Tertiary: 
Red  and  light-gray  conglomerate  and  sandstone. 
White  clay,  more  or  less  calcareous  and  commonly  sandy  (White  River  formation). 
Fort  Union  (?)  formation  (shales,  sandstones,  and  coal). 
Cretaceous: 
Montana — upper  two-thirds  principally  sandstone  with  coal;  lower  third  dark 
shale  with  no  coal. 
Colorado — mostly  dark-brown  shale  with  some  brown  sandstone  layers,  and  at  the 
top  a  buff  to  white  sandstone,  which  maybe  Niobrara.  Near  the  base  is  the 
Mowry  member,  a  very  resistant  dark  shale  which  weathers  white. 
Cloverly — massive,  brown,  resistant  sandstone,  which  commonly  forms  a  pro- 
nounced hogback.  Near  the  top  there  is  some  shale,  and  at  the  bottom  a  fine 
conglomerate. 
