COAL  IN  NORTHWESTERN  COLORADO  AND  NORTHEASTERN  UTAH.       287 
Rock  formations  of  the  northwestern  Colorado  coalfields — Continued. 
C.eo- 
logic 
age. 
Forma- 
tion 
Description  of  strata. 
Consists  of  massive 
sandst  ones  and 
light-colored  sandy 
or  clay  shales  and 
many  beds  of  lig- 
nite or  subbitumi- 
nouscoal.  As 
mapped,  this  for- 
mation includes  the 
strata  in  which  the 
massive  sand- 
stones are  most 
prominent  as  dis- 
tinguished from 
the  softer  strata  of 
the  underlying 
Lewis  shale.  As  a 
whole  the  forma- 
tion represents  the 
transitional  depos- 
its of  brackish  or 
fresh-water  origin, 
conformably  over- 
lying the  upper- 
most marine  for- 
mation. The  low- 
ermost sandstones 
mapped  with  the 
Laramie  contain  a 
marine  fauna  simi- 
lar to  that  of  the 
underlying  Lewis 
shale. 
Topographic  feal  ures. 
In  the  eastern  part  of 
the  Yampa  River 
valley  the  Laramie 
is  clearly  distin- 
guished from  the 
underlying  Lewis 
both  iithologically 
and  in  its  effect  on 
the  topography.  It 
is  similar  to  the 
overlying  and  sup- 
posedly lower  Ter- 
tiary beds,  from 
which  it  is  distin- 
guished only  by  the 
presence  of  the  in- 
tervening conglom- 
erate, and  evidence 
of  unconformity. 
In  the  western  part 
of  the  region  the 
lower  limit  of  the 
Laramie  is  less 
clearly  defined,  as 
the  sandstones 
probably  occur  low- 
er than  they  do  far- 
ther east.  The  base 
of  the  formation  is 
well  represented  at 
Hay  den,  in  the  es- 
cafpment  bluff 
north  of  Yampa 
River. 
Thickness. 
Estimated  as  1,200 
feet  between 
Craig  and  Lay. 
Economic  value. 
Workable  coal. 
Composed  largely  of 
soft  dark-gray  or 
black  clay  shale 
with  calcareous 
seams  and  sandy 
beds,  the  latter  de- 
veloping in  many 
places  as  massive 
and  continuous 
ledges.  The  ab- 
sence of  the  sand- 
stones is  the  most 
prominent  distinc- 
tion between  the 
Lewis  and  the  over- 
lying and  underly- 
ing formations. 
The  outcrop  of  the 
Lewis  shale  is  com- 
monly marked  by 
low  valleys  bound- 
ed on  one  side  by 
the  sandstone 
ridges  of  the  Mesa- 
verde  and  on  the 
other  by  the  escarp- 
ments of  the  Lara- 
mie. In  the  west- 
ern part  of  the 
Yampa  field  the  dis- 
tinction is  not  so 
well  marked,  as 
sandstone  occurs 
within  the  interval 
assumed  to  repre- 
sent the  Lewis. 
Roughly  estima- 
ted 1,000  feet  in 
the     valley    of  . 
Horse  Gulch. 
Develops  some  val- 
uable agricul- 
tural land  on  al- 
luvial material 
along  the  valley 
bottoms. 
Includes  an  alternat- 
ing succession  of 
sandstone,  sandy 
shale,  and  coal 
beds.  The  most 
prominent  mem- 
bers are  the  mas- 
sive white  or  light- 
colored  sandstones. 
The  weaker  shary 
members  are  more 
commonly  coven  d 
by  del  iris  of  the 
harder  beds.  The 
coal  beds  are  even 
more  generally  con- 
cealed, although 
their  positions  are 
rendered  conspicu- 
ous by  the  great, 
amount  of  burning 
that  has  taken 
place  along  the 
outcrop. 
Formshogbaek  ridges 
and  rugged,  hilly 
country.  The  to- 
pographic forms  are 
usually  strong,  con- 
trasting  with  the 
valleys  of  overlying 
and  underlying' for- 
mations. Thechar- 
acter  of  the  ridges 
is  chiefly  dependent 
(Hi  the 'attitude  of 
the  underlying  stra- 
ta. Where  steeply 
tilted  the  harder 
beds  form  sharp- 
crested  hogbacks, 
and  where  more 
nearly  horizontal 
they  form  broader 
flat  or  round-top- 
ped ridges  border- 
ed by  escarpments 
or  long,  gentle  dip 
slop.  s. 
From  about  :b0( K) 
to  over  5,000 
feet. 
Coal.  This  forma- 
tion commonly 
includes  many 
thick  beds  of 
good  bitumin- 
ous coal. 
