SENTINEL   BUTTE   LIGNITE   FIELD,   N.   DAK.    AND    MONT.  25 
well  record  shows  it  to  be  23  feet  thick  and  about  120  feet  below  the 
surface. 
A  lignite  occupying  the  same  position  in  the  section  as  the  Har- 
mon bed  outcrops  in  the  east  bank  of  Beaver  Creek,  2  miles  north  of 
Wibaux,  Mont.  It  ranges  from  8  feet  6  inches  to  9  feet  3  inches  in 
thickness  and  is  mined  to  supply  local  trade.  Northward  from  this 
point  the  lignite  dips  below  the  level  of  the  creek,  but  to  the  south 
it  rises  slightly  above  the  creek  level  as  far  as  observed.  In  T.  15  N., 
R.  59  E.,  in  Montana,  the  Harmon  lignite  outcrops  near  the  head  of 
Box  Elder  Creek,  a  tributary  of  Yellowstone  River,  but  its  outcrop 
has  not  been  mapped.  In  T.  16  N.,  R.  59  E.,  this  bed  outcrops 
near  the  head  of  Cottonwood  Creek,  but  owing  to  the  fact  that  it 
has  been  burned  extensively  no  complete  section  of  the  bed  could 
be  measured.  A  partial  exposure,  where  only  the  top  portion  could 
be  seen,  measures  8  to  10  feet.  The  upper  part  of  this  bed,  or  a  bed 
occupying  a  similar  position  in  the  section,  was  noted  in  the  south 
bank  of  Beaver  Creek,  a  short  distance  north  of  the  field. 
MEDORA    GROUP   OF   LIGNITE    BEDS. 
The  Medora  group  of  lignite  beds,  so  named  because  of  the  occur- 
rence and  exposure  of  its  members  in  the  bluffs  of  Little  Missouri 
River  at  and  near  Medora,  N.  Dak.,  occupies  a  position  from  150  to 
290  feet  above  the  Harmon  bed.  The  group  comprises  four  beds, 
designated  in  the  vertical  section  on  the  map  by  the  letters  B,  C,  D, 
and  E.  Along  Little  Missouri  River  the  outcrop  of  only  one  mem- 
ber of  the  group,  bed  C,  has  been  mapped.  This  bed,  the  second 
above  the  base  of  the  group,  is  more  persistent  and  thicker  than  the 
others,  and  has  been  mined  for  local  use  at  a  number  of  places, 
notably  near  Medora.  But  as  the  three  lower  members  lie  within 
a  vertical  range  of  60  to  80  feet,  the  mapped  outcrop  of  the  second 
above  the  base,  bed  C,  marks  approximately  in  steep  slopes  and 
bluffs  the  outcrop  of  the  other  two.  The  outcrop  of  the  uppermost 
member  of  the  group  has  been  mapped  wherever  it  shows  workable 
thickness. 
At  the  southern  boundary  of  T.  139  N.,  R.  102  W.,  the  lowest 
member  of  the  Medora  group,  bed  B,  has  an  elevation  of  100  feet 
above  the  level  of  Little  Missouri  River,  and  in  conformity  with  the 
general  dip  of  the  inclosing  rocks  the  various  members  of  the  group 
dip  toward  the  northeast  and  disappear  below  the  level  of  the  river 
near  the  north  line  of  T.  140  N.,  R.  102  W. 
All  the  members  of  the  group  are  exposed  in  the  northern  and 
western  parts  of  the  field,  notably  on  Elk,  Beaver,  Little  Beaver,  and 
Dry  creeks. 
