COAL    NEAR   CKAZY    MOUNTAINS,    MONIW  81 
Overlying  the  Colorado  shale  is  the  Montana  group,  composed  of 
four  formations — the  Eagle,  Claggett,  Judith  River,  and  Bearpaw,  all 
of  which  are  of  Cretaceous  age.  The  Eagle  sandstone,  composed  very 
largely  of  massive  gray  sandstone,  is  from  100  to  250  feet  thick  and 
presents  a  marked  contrast  to  the  underlying  dark  Colorado  shale. 
One  of  the  coal  horizons  of  this  region  is  in  the  Eagle  sandstone,  but 
the  bed  is  not  of  workable  thickness.  This  sandstone  is  a  conspicuous 
ledge  maker  and  for  that  reason  and  because  of  its  whiteness  and 
tendency  to  weather  in  stools  is  easily  traced. 
The  Claggett  formation  where  well  exposed  produces  as  a  whole 
a  light-brown  or  tan  effect  with  pronounced  ribbing  due  to  thin  beds 
of  hard  sandstone  in  the  shale  that  makes  up  the  greater  part  of  the 
formation.  It  is  a  marine  and  brackish-water  deposit  and  carries 
abundant  invertebrate  fossils.  Its  thickness  ranges  from  400  to  800 
feet.  The  top  member  of  the  Claggett  formation  is  a  massive  gray 
sandstone  which  in  many  places  makes  a  conspicuous  cliff.  It  closely 
resembles  the  massive  Eagle  sandstone  and  may  be  readily  mistaken 
for  it. 
The  Judith  River  formation,  lying  next  above  the  Claggett  and 
having  about  the  same  range  in  thickness,  is  prevailingly  sandy. 
When  tilted  it  forms  a  low  ridge  and  appears  dark  gray;  where 
horizontal  it  tends  to  weather  in  badland  forms  and  is  banded  with 
light-colored  beds.  Near  the  head  of  Musselshell  River  the  top  bed 
of  the  Judith  River  formation  is  a  sandstone  which  ranges  up  to 
15  feet  in  thickness  and  is  full  of  oyster  shells;  in  places,  especially 
around  Gordon  Butte,  the  bed  is  practically  a  solid  mass  of  shells, 
8  feet  thick.  Seams  of  coal  a  few  inches  thick  are  known  to  occur 
in  this  formation,  which  was  laid  down  mainly  in  fresh  water,  with 
an  occasional  recurrence  of  brackish  conditions.  Besides  shells  it 
contains  fossil  bones  and  wood. 
Above  this  mass  of  prevailingly  fresh-water  beds  are  strata  ranging 
in  thickness  from  700  to  1,100  feet  and  composed  almost  entirely  of 
dark-gray  and  greenish  shale.  The  color  when  wet  is  nearly  black, 
and  contrasts  strongly  with  that  of  the  formations  above  and  below. 
This  shale  constitutes  the  upper  part  of  the  Montana  group  and  is 
termed  the  Bearpaw  shale.  It  is  so  thick  and  weathers  so  readily 
that  where  it  lies  flat  it  forms  an  area  of  very  low  relief,  and  where 
it  is  tilted  a  valley  is  developed  in  it  between  ridges  of  the  harder 
Judith  River  formation  below  and  the"  red  sandstones  above.  A 
feature  of  some  note  is  the  occurrence  of  limy  concretions  about  a 
foot  in  diameter,  which  are  found  commonly  in  the  upper  part  of  the 
formation  and  which  are  as  a  rule  highly  fossiliferous. 
The  red  and  greenish  sandstones  overlying  the  Bearpaw  shale 
constitute  a  distinct  lithologic  unit,  ranging  in  thickness  from  200 
to  460  feet,  and,  although  containing  some  shale  members,  are 
71  197— Bull.  ::il     09        <: 
