THE  NORTH  DAKOTA-MONTANA  LIGNITE  AREA. 
By  A.  G.  Leonard. 
LOCATION. 
The  North  Dakota-Montana  lignite  area  includes  most  of  the  western  half  of  North 
Dakota,  eastern  Montana,  and  the  adjoining  parts  of  South  Dakota  and  Wyoming;  it  also 
extends  north  into  Canada.  Lignite  is  known  to  occur  as  far  west  as  Bighorn  River  and  1 
the  mouth  of  Milk  River  in  Montana  and  the  Bighorn  Mountains  in  Wyoming,  or  about 
150  miles  west  of  the  North  Dakota  line.  In  South  Dakota  the  lignite  is  confined  to  the 
northwest  corner  of  the  State.  Farther  west  the  lignite  field  extends  south  into  Wyo- 
ming, where  it  includes  portions  of  Sheridan,  Johnson,  Crook,  and  Weston  counties.  The 
area  has  an  extent  of  about  350  miles  east  and  west  and  at  its  widest  part  nearly  the  same 
extent  north  and  south,  containing  approximately  100,000  square  miles.  This  is  over 
twice  the  area  of  New  York  State  and  is  about  equal  to  that  of  Colorado.  North  Dakota 
and  Montana  each  includes  about  40,000  square  miles  of  the  total  area. 
TOPOGRAPHY. 
The  topography  of  by  far  the  greater  portion  of  this  extensive  region  is  that  of  a  gently 
rolling  or  rough  plain  which  has  an  elevation  of  from  2,000  to  nearly  3,000  feet  above  sea 
level.  Rising  above  the  general  level  of  this  plain  and  forming  conspicuous  features  of 
the  landscape  are  numerous  flat-topped  buttes  and  hills.  Slim  Buttes  and  Cave  Hills  in 
South  Dakota  and  Sentinel  Butte,  Bullion  Butte,  and  the  Killdeer  Mountains  in  North 
Dakota  are  notable  examples.  They  rise  abruptly  from  400  to  600  feet  above  the  plain. 
The  buttes  are  especially  characteristic  of  the  eastern  part  of  the  region,  in  North  and 
South  Dakota. 
On  the  northern  and  eastern  borders  of  the  lignite  area  the  surface  has  been  modified 
to  a  considerable  extent  by  the  continental  ice  sheet,  which  left  behind  a  mantle  of  bowlder 
clay,  sand,  and  gravel  that  forms  a  gently  rolling  plain.  This  drift  plain  is  found  east  and 
north  of  Missouri  River  and  extends  .50  miles  and  more  south  and  west  of  that  stream. 
A  striking  contrast  to  the  flat,  wide  prairies  is  furnished  by  the  extremely  rough  bad- 
land  areas  bordering  the  streams  and  frequently  extending  back  from  them  many  miles. 
The  bad  lands  are  perhaps  best  developed  along  Little  Missouri  and  Yellowstone  rivers, 
though  they  are  by  no  means  confined  to  the  vicinity  of  these  streams. 
The  lignite  area  is  traversed  by  several  rivers  of  importance,  whose  broad,  deep  valleys 
form  conspicuous  topographic  features,  besides  affording  favorable  routes  for  railroads. 
Those  of  the  Missouri  and  Yellowstone  are  the  most  extensive,  being  from  200  to  300  feet 
deep  and  from  1  mile  to  several  miles  wide;  of  less  importance  are  the  valleys  of  Little 
Missouri,  Tongue,  and  Powder  rivers. 
STRATIGRAPHY. 
Pierre  shale. — All  the  rocks  exposed  in  the  lignite  area  belong  to  the  Cretaceous  system. 
Those  immediately  underlying  the  coal-bearing  beds  belong  to  the  Pierre  shale,  which  also 
borders  the  "  Laramie  "  a  or  coal-bearing  formation  on  the  east  and  south.     The  bluish-gray, 
•a  In  conformity  with  common  usage  in  the  North  Dakota  field,  the  term  Laramie  is  here  used  for 
the  lignite-bearing  beds,  although  it  is  recognized  that  the  name  Fort  Union  may  be  more  appropriate. 
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