LEWISTOWN    COAL   FIELD,   MONTANA.  100 
Falls  areas."  The  territory  investigated  lies  between  longitude  109° 
and  110°  15'  W.,  and  the  47th  parallel  divides  it  into  two  nearly 
equal  portions.  It  is  thus  located  in  the  center  of  Montana,  includ- 
ing the  west-central  part  of  Fergus  County  and  a  few  square  miles 
of  northeastern  Meagher  County.  The  greater  part  of  the  field 
lies  in  the  Judith  Basin,  a  name  applied  to  the  upper  drainage  area 
of  Judith  River,  but  it  also  includes  a  strip  along  the  headwaters 
of  McDonald  Creek  and  a  portion  of  the  Great  Plains  region  lying 
east  of  Judith  Mountains.  The  Little  Belt  and  Big  Snowy  moun- 
tains are  in  the  southern  part  of  the  field  and  the  Judith  and  South 
Moccasin  groups  lie  well  within  its  northern  borders.  The  area  as 
described  includes  about  1,500  square  miles. 
TOPOGRAPHY. 
The  topography  of  the  field  is  somewhat  diversified.  In  the 
western  half  the  surface  features  are  essentially  those  of  a  plains 
region,  which  is  but  slightly  dissected.  Judith  River  and  its  main 
tributary  from  the  south,  Ross  Fork,  flow  through  this  district  and, 
with  their  branches,  have  accentuated  the  surface  of  the  structural 
basin  formed  by  the  encircling  uplifts  of  the  Little  Belt,  Big  Snowy, 
Judith,  and  South  Moccasin  mountains.  The  Little  Belt  Range, 
bordering  the  southwestern  part  of  the  field,  presents  a  bold  north- 
ern face,  and  there  is,  in  consequence,  but  a  narrow  transitional  zone 
between  plains  and  mountain  topography  in  that  district. 
In  contrast  to  the  Little  Belt  Range,  the  Big  Snowy  Mountains 
recede  to  the  north  in  long  slopes  which  are  deeply  trenched  by 
streams.  This  dissected  surface  is  characteristic  of  much  of  the 
eastern  part  of  the  field,  especially  in  the  vicinity  of  the  connecting 
ridge  between  the  Big  Snowy  and  Judith  mountains,  which  will  be 
referred  to  in  this  report  as  the  McDonald  Creek  divide.  The  Judith 
and  Moccasin  mountains  are  fairly  rugged  and  rise  rather  abruptly 
from  the  plains.  Higher  peaks  in  the  Judith  uplift  approximate 
6,000  feet  in  altitude;  those  of  the  Moccasin  Mountains  are  somewhat 
lower.  The  greatest  altitude  is  in  the  Little  Belt  Range,  where  two 
summits  reach  7,400  feet  above  sea  level.  The  plains  range  in 
altitude  from  3,600  to  5,000  feet. 
In  general  the  topography  of  the  field  favors  development  of 
the  coal,  as  the  streams  cut  across  the  coal  zone  in  many  localities, 
allowing  access  to  the  outcrop,  and  the  valleys  serve  as  natural  routes 
for  transportation.  The  broad  terraces  in  the  western  part  of  the 
field  afford  easy  routes  for  railroads,  and  as  a  result  the  Montana 
Railroad  reaches  Lewistown  from  the  southwest  through  Judith 
Gap  with  scarcely  a  cut  or  fill  within  the  field. 
a  Fisher,  C.  A.,  The  Great  Falls  coal  field,  Montana:  BulL  U.  S.  Geol.  Sin  6,1907,pp.  L61-173 
