YAMPA    COAL    FIELD,    COLORADO. 
disappearing  into  an  area  of  igneous  eruptives  in  the  direction  of  Wolf  Mountain.  This 
cross  buckle  of  the  strata  lies  approximately  at  right  angles  to  the  major  axis  and  cuts  off  a 
structural  basin  on  the  southeast  or  Yampa  end  of  the  field.  The  center  of  this  subordinate 
basin  is  marked  by  the  open  prairie  land  of  Twentymile  Park. 
The  rest  of  the  field  is  a  broadly  opened  syneline  whose  outer  rim  is  made  up  of  ridges 
formed  by  the  massive  sandstones  of  the  coal-bearing  secies.  Within  this  rim  which  is  a 
marked  topographic  feature,  is  a  belt  of  open  prairie  and  valley  land  eroded  on  the  soft 
black  shale  that  overlies  the  coal  beds.  These  in  turn  are  replaced  in  the  center  of  the 
syneline  north  and  west  of  the  town  of  Hayden  by  beds  of  the  overlying  Laramie  formation. 
The  latter  dip  away  to  the  northwest  until  they  are  covered  by  the  overlying  Tertiary  rocks, 
which  are  supposed  to  cover  all  of  the  region  along  the  Colorado  and  Wyoming  line. 
West  of  Twentymile  Park  the  sandstone  ridges  swing  to  the  west,  and  form  a.  low  range  of 
mountains  on  the  north  side  of  Williams  Fork,  called  the  Williams  Fork  Mountains.  The 
sharply  dissected  southern  slope  of  these  mountains  exposes  the  upturned  edges  <>l  the 
whole  coal  series  for  over  20  miles.  The  coal  beds  dip  to  the  north,  passing  under  the  barren 
shale  which  forms  the  valley  of  Yampa  River. 
The  rim  of  sandstone  ridges  bounds  the  east  side  of  Hayden  Valley  from  near  Twentymile 
Park  to  Wolf  Mountain  and  Pilot  Knob  of  the  Anthracite.  Range.  North  of  Pilot  Knob  the 
sandstones  are  much  broken  by  intrusions  of  sheets  and  dikes  of  basaltic  lava  and  conse- 
quently the  rim  of  the  basin  is  not  well  marked,  but  the  coal  beds  have  been  traced  in  this 
direction  as  far  as  the  flanks  of  Sand  Mountain.  All  along  this  eastern  rim  of  the  field  the 
rocks  have  a  general  westerly  dip,  also  passing  under  the  barren  shale  of  the  valley  of  Yampa 
River.  In  all  probabilit}?-  the  coals  of  the  eastern  rim  extend  beneath  Hayden  Valley  and 
are  continuous  with  the  coal  beds  showing  in  the  Williams  Fork  Mountains. 
STRATIGRAPHY. 
On  the  eastern  side  of  the  great  syneline  strata  as  old  as  Triassic  He  tilted  against  the 
Archean  core  of  the  Park  Range.  To  the  west  the  Dakota,  Benton,  and  Niobrara  are 
crossed,  each  in  its  turn  dipping  under  the  next  higher  formation.  Above  these,  at  a 
distance  of  several  miles  from  the  Archean  core1,  is  a,  thick  mass  of  coal-bearing  strata. 
These  rocks  have  been  previously  considered  as  of  Laramie  age,  but  fossils  collected  from 
them  during  the  past  season's  work  were  sufficient  to  determine  definitely  that  they  belong 
to  the  Montana  group  of  the  Cretaceous.  This  group  is  the  equivalent  of  the  Pierre  and 
Fox  Hills  formations  of  other  fields,  although  this  subdivision  can  not  well  be  applied  here. 
Stratigraphic  sections  in  different  parts  of  the  field  do  not  agree  in  their  minor  details, 
but  a  few  of  the  dominant  members  of  the  series  are  very  persistent  throughout  the  area 
and  serve  as  useful  horizon  markers.  The  chief  of  these  are  some  of  the  massive  sandstone 
beds  and  the  three  groups  of  coal  seams.  The  following  is  a  generalized  section,  compiled 
from  many  such  sections  measured  in  various  parts  of  the  region: 
Generalized  section  of  the  coal-hearing  series. 
Laramie:  Feet. 
Alternating  soft  sandstone  and  shale,  with  frequent  lignite  seams 900+ 
Montana: 
(1)  Dark-colored  clay  shale,  with  calcareous  seams,  estimated  as 1,000  2,000 
CI)  Coal-bearing  rocks,  consisting  of— 
(a)  Alternating  sandstone  (at  places  massive)  and  shale  containing  the  upper 
group  of  coals 400 
(/i)  A  persistent  massive  white  sandstone •r,,)-     ' 50 
(c)  Alternating  sandstone  and  shale  beds  containing  in  their  lower  half  the  mid- 
dle group  of  coals >  .000 
(d)  A  persistent  massive  white  sandstone 50-     l,n 
(c)  Alternating  sandstone  and  shale  beds  containing  near  their  middle  the  lower 
group  of  coals 
(/)  Alternating  sandstone  and  shale  beds,  massive  sandstone  predominating, 
with  a  few  small  coal  seams  near  its  base 
(3)  Dark-colored  clay  shale -  ■  1,000-1,500 
1,000-7,700 
