SENTINEL   BUTTE   LIGNITE  FIELD,   N.   DAK.   AND   MONT.  21 
Ft.    in. 
Clay,  gray  and  yellow,  sandy 53 
Lignite 6 
Sand,  fine  grained,  clayey 12 
Clay,  brown  and  gray,  containing  many  selenite  crystals 4 
Sandstone,  soft,  fine-grained 1 
Lignite 1     6 
Clay,  brown  and  carbonaceous 1 
Clay,  bluish  gray 10 
Sand,  gray,  cemented  in  places  into  a  soft  sandstone 12 
Clay  and  sand ,  not  well  exposed 55 
Lignite 6 
Clay,  gray,  sandy 37 
Clay,  gray,  with  no  sand 2 
Lignite 6 
Clay,  brown  at  the  top,  sandy 5 
Sand,  gray,  fine 4 
Clay,  gray,  sandy,  containing  nodules 15 
Sand ,  finely  laminated 4 
Clay,  gray,  sandy,  with  ferruginous  bands 8 
Clay,  brown,  sandy 1 
Clay,  gray 5 
Clay,  gray,  sandy,  containing  abundant  siliceous  and  ferruginous 
nodules,  arranged  mostly  in  bands  at  certain  horizons;  these 
hard  nodules  project  from  surface  of  softer  clays,  and  cap  small 
clay  columns 25 
Sand  and  clay  not  well  exposed 25 
Lignite 21    2 
Unexposed  to  level  of  railroad  at  Sentinel  Butte  station 190 
650    2 
The  Fort  Union  formation  has  a  thickness  in  this  region,  as 
measured  on  its  outcrop,  of  900  feet.  If  to  this  be  added  820  feet 
of  lignite-bearing  rocks,  which  are  probably  to  be  referred  to  the 
Fort  Union,  penetrated  by  the  Medora  well  below  the  lowest  lignite 
outcropping  in  the  field,  the  total  thickness  is  1,720  feet. 
OLIGOCENE    ROCKS. 
Overlying  the  heavy  sandstone  which  forms  the  summit  of  Sentinel 
Butte  and  constitutes  the  topmost  member  of  the  Fort  Union  forma- 
tion, there  are  about  40  feet  of  calcareous  clay  and  limestone,  as 
shown  in  the  Sentinel  Butte  section.  These  beds  are  merely  the 
remains  of  a  formation  which  doubtless  at  one  time  covered  a  large 
area  in  this  region.  Strata  which  have  yielded  Oligocene  vertebrates, 
and  which  occupy  a  similar  horizon  immediately  over  the  massive 
sandstones  at  the  top  of  the  Fort  Union,  occur  in  Chalk  Butte,  70 
miles  farther  southeast.  Other  similar  buttes  in  northwestern  South 
Dakota  and  southeastern  Montana  have  likewise  been  referred  to  the 
Oligocene. 
