COAL  ON   NORTHEAST  SIDE   OF  BIGHORN   BASIN,   WYO.  169 
State  line.  At  that  place  fossils  of  the  Pierre  fauna  (marine)  were 
collected  from  calcareous  nodules  in  black  shale. 
Laramie  formation. — The  Laramie  formation  consists  of  150  to  700 
feet  of  massive  sandstone  with  subordinate  shale.  The  sandstones 
are  thick,  coarse  grained,  and  conspicuous,  and  contain  many  large 
ferruginous  concretions.  Usually  the  massive  sandstone  at  the  top  of 
the  Laramie  is  white  from  leaching  or  other  causes.  In  the  Basin 
field  the  lower  part  of  the  formation  contains  100  feet  of  varicolored 
soft  shale — dark  green,  white,  black,  and  other  colors — in  which  there 
are  three  closely  adjacent  coal  beds.  The  middle  bed  is  workable.  In 
the  Garland  field  three  or  more  coal  beds  occur  between  300  and  400 
feet  above  the  base  of  the  Laramie,  above  a  series  of  massive  sand- 
stones and  at  the  base  of  several  hundred  feet  of  dark  shale. 
Fort  Union  formation.— -The  contact  between  the  Fort  Union  and 
the  Laramie  is  probably  unconformable,  but  the  unconformity  is  not 
apparent  in  most  places,  and  the  formations  are  hard  to  separate  in 
the  field.  Lithologically  they  are  almost  identical.  The  Fort  Union 
consists  principally  of  dark  shale  with  thick  lenticular  masses  of  sand- 
stone. The  sandstone  is  especially  abundant  near  the  bottom  of  the 
formation,  where  some  of  the  beds  are  in  places  more  than  100  feet 
thick.  West  of  Sheep  Mountain  there  is  over  1,200  feet  of  white 
conglomeratic  sandstone  at  the  base  of  the  Fort  Union,  but  this  is 
exceptional.  The  lenticular  sandstones  in  the  formation  contain 
many  ferruginous  concretions,  which  reach  40  feet  or  more  in  diam- 
eter. The  shale  contains  smaller,  hard,  dense,  ferruginous  concre- 
tions, weathering  yellow  and  brown.  These  are  characteristic  of  the 
formation.  Coal  occurs  locally  in  all  parts  of  the  Fort  Union  forma- 
tion. The  coal  beds  mined  southwest  of  Basin,  near  Manderson,  are 
about  700  feet  above  its  base. 
Wasatch  formation. — The  Wasatch  formation  is  a  series  of  varie- 
gated red  and  pink  banded  shales  or  clays,  containing  subordinate, 
irregular,  nonpersistent  beds  of  soft  white  sandstone.  It  rests  in 
places  unconformably  on  the  upturned  edges  of  the  underlying  for- 
mations; in  other  places  the  unconformity  is  not  conspicuous.  The 
Wasatch  does  not  contain  coal. 
STRUCTURE. 
In  general,  the  structure  of  the  coal-bearing  rocks  is  that  of  a 
simple  westward  or  southwestward  dipping  monocline,  forming  part 
of  the  east  limb  of  the  Bighorn  Basin  syncline.  The  angle  of  dip  is 
low  (2°  to  20°  SW.)  at  all  places  where  the  coal  is  of  workable  thick- 
ness. West  of  Sheep  Mountain  the  Laramie  coal  beds  are  nearly 
vertical,  but  they  have  no  economic  value.  In  the  cent  ml  part  of 
the  basin  the  coal  beds  are  doubtless  flat,  but  they  arc  covered  by 
several  thousand  feet  of  barren  rock. 
