SENTINEL   BUTTE   LIGNITE    FIELD,   N.    DAK.    AND    MONT. 
north  to  south  along  its  eastern  border  only.  On  the  west  they  pass 
into  the  rolling  plain  which  stretches  to  the  badlands  of  Yellowstone 
River. 
Conspicuous  features  of  the  valley  of  the  Little  Missouri  are  the 
broad  flats  which  occur  at  intervals  on  both  sides  at  an  elevation  of 
240  feet  above  the  river.  They  have  a  width  of  1  or  2  miles  and  over- 
looking them  are  bluffs  rising  rather  abruptly  160  to  200  feet  higher, 
or  about  420  feet  above  the  river.  These  flats  were  probably  formed 
when  the  land  was  considerably  lower  than  at  present,  and  the  river, 
having  reached  base-level,  meandered  back  and  forth  over  a  flood 
plain  4  or  5  miles  wide.  The  surface  was  then  elevated,  and  the  river 
gained  a  new  erosive  power  and  has  since  cut  its  inner  gorge  to  a  depth 
of  about  240  feet  below  its  old  valley  bottom,  represented  by  the  flats. 
The  valley  as  a  whole  has  a  depth  of  420  to  440  feet  below  the  upland 
plain  in  which  it  has  been  eroded.  The  altitude  of  this  upland  plain 
ranges  from  2,700  to  2,800  feet,  and  rising  above  it  are  three  prominent 
buttes — Sentinel,  Square,  and  Camels  Hump — with  great  numbers  of 
lesser  buttes.  Sentinel  Butte  has  a  height  of  3,350  feet  above  sea 
level,  and  its  flat  summit  forms  a  landmark  visible  for  20  to  30  miles 
in  all  directions. 
The  gently  rolling  plain  is  itself  the  product  of  long-continued  ero- 
sion, which  has  doubtless  been  going  on  ever  since  Oligocene  time, 
resulting  in  the  removal  of  800  to  1,000  feet  of  strata  from  this  region. 
Remnants  of  these  eroded  beds  are  found  in  the  buttes  just  mentioned, 
Sentinel  Butte  rising  650  feet  above  the  plain  at  its  base  and  contain- 
ing on  its  summit  the  evidence  of  a  still  more  recent  formation  that  has 
almost  entirely  disappeared.  Sentinel  and  Square  buttes  are  capped 
with  heavy  ledges  of  sandstone,  about  100  feet  thick,  which  have  pro- 
tected them  from  erosion  and  given  them  their  flat  summits.  In  the 
vicinity  of  Sentinel  Butte  there  are  a  large  number  of  small  buttes  that 
rise  from  150  to  175  feet  above  the  surrounding  surface  to  a  fairly  uni- 
form height  and  are  capped  by  a  thick  layer  of  baked  and  fused  clay. 
This  red  clay,  which  has  protected  the  buttes  from  erosion,  was  formed 
by  the  burning  of  the  21-foot  bed  of  lignite  that  is  present  in  the  base 
of  Sentinel  Butte. 
In  places  the  change  is  abrupt  from  the  upland  plain  to  the  badland 
belt  along  the  river.  To  one  looking  out  over  the  badlands  from  some 
commanding  point  the  scene  presents  an  indescribable  waste  of  steep- 
sided  ridges,  bluffs,  buttes,  domes,  and  pinnacles  carved  out  of  the  soft 
clays  and  sands  of  the  Fort  Union  formation.  The  bare  slopes  which 
are  so  conspicuous  a  feature  of  the  region  everywhere  show  the  marks 
of  the  last  shower,  being  grooved  with  countless  tiny  channels  formed 
by  the  rivulets  of  water  which  poured  down  their  sides.  At  one  or 
more  horizons  bands  of  red,  burnt  clay  may  be  readily  traced  with  the 
eye  for  many  miles  in  the  bluffs  and  ridges.     These  beds  of  clay, 
71497— Bull.  341—09 2 
