CONTRIBUTIONS   TO   ECONOMIC    GEOLOGY,   1907,   PART   II. 
STRUCTURE. 
Little  can  be  said  of  the  structure  of  the  small  area  here  discussed 
except  that  the  strata  have  a  low  dip — 20  feet  to  the  mile — toward 
the  northeast.  However,  work  in  adjoining  territory  has  proved 
that  the  structure  of  the  field  is  the  result  of  an  upward  fold  of  the 
rocks,  or  anticline,  whose  axis  trends  northwest  and  southeast  and  is 
located  25  miles  to  the  southwest.  This  anticline  is  well  exposed  at 
the  mouth  of  Cedar  Creek,  Montana,  where  it  is  crossed  by  Yellow- 
stone River.  Here  300  feet  of  Pierre  shale,  of  Cretaceous  age,  is 
brought  to  the  surface,  and  the  overlying  rocks,  including  the  Fort 
Union,  dip  away  from  it,  steeply  toward  the  southwest  and  gently 
toward  the  northeast.  The  gentle  dip  continues,  however,  to  the 
northeast  corner  of  the  field  represented  on  the  accompanying  map, 
where  the  rocks  are  horizontal.  What  is  probably  an  extension  of 
this  disturbance  is  marked  by  an  exposure  of  Pierre  shale  on  Little 
Beaver  Creek,  in  Bowman  County,  N.  Dak.a 
Throughout  the  field  the  dip  is  so  slight  that  it  can  be  detected  only 
by  tracing  certain  well-marked  horizons  and  by  noting  their  change 
of  altitude  from  point  to  point.  For  example,  the  elevation  of  the 
contact  of  the  buff  and  somber-colored  members  in  the  base  of  Sentinel 
Butte  is  2,850  feet  above  sea  level,  but  in  the  northeast  corner  of  the 
area  the  same  contact  is  at  2,430  feet.  Further  evidence  that  the 
rocks  are  tilted  is  the  fact  that  artesian  water  has  been  obtained  at 
Medora.  A  well  was  sunk  here  by  the  Northern  Pacific  Railway 
Company  to  a  depth  of  940  feet,  where  a  water-bearing  sandstone 
was  reached.  To  judge  from  the  thickness  of  the  Fort  Union  section 
at  Glendive,  Mont.,  it  is  believed  that  this  sandstone  lies  near  the  base 
of  the  formation  and  that  it  is  exposed  at  a  higher  level  far  to  the 
southwest  along  the  flank  of  the  anticline  mentioned  above. 
THE  LIGNITE. 
GENERAL  STATEMENT. 
By  referring  to  the  columnar  section  on  the  map  accompanying 
this  report  (PL  II),  it  will  be  noted  that  workable  beds  of  lignite 
occur  at  nine  different  horizons  in  the  900  feet  of  strata  exposed  in 
the  field.  A  great  many  other  beds  1  foot  or  less  in  thickness  can  be 
seen  in  almost  any  well-exposed  rock  section,  but  no  attempt  has 
been  made  to  note  them.  The  workable  lignite  beds,  with  one 
exception,  are  for  convenience  of  description  divided  into  groups,  the 
lower  being  termed  the  Medora  and  the  upper  the  Sentinel  Butte 
group.  The  lowest  lignite  exposed  in  the  field  is  called  the  Harmon 
bed;  it  outcrops  in  the  east  bank  of  Little  Missouri  River  a  short 
a  Leonard,  A.  G.,  Bull.  U.  S.  Geol.  Survey  No.  316, 1907,  p.  196. 
