COAL  IN  NORTHWESTERN  COL<  RADO  AND  NORTHEASTERN   UTAH.        285 
GEOLOGY. 
STRATIGRAPHY. 
SIGNIFICANCE    OF    THE    ROCK    FORMATIONS. 
The  deposits  in  which  the  coal  was  originally  formed  are  a  part  of 
a  great  series  of  sedimentary  strata  that  were  laid  down  at  a  time 
when  this  part  of  the  continent  was  largely  submerged.  The  sub- 
mergence probably  ranged  from  deep  sea  to  shallow  water  and 
swampy  conditions,  as  well  as  from  salt-water  or  marine  to  brackish 
and  fresh  water  stages.  These  strata  were  deposited  in  an  approxi- 
mately horizontal  position.  Subsequent  movements  of  the  earth's 
crust  have  folded  or  even  broken  the  originally  continuous  deposits, 
so  that  it  is  now  necessary  to  study  their  attitude  and  order  of  super- 
position in  order  to  explain  the  relations  of  one  set  of  beds  to  another. 
Their  position  in  the  stratigraphic  section  indicates  in  a  general  way 
their  relative  geologic  age,  although  there  are  also  evidences  that  great 
time  intervals  have  elapsed  in  winch  no  deposits  were  being  formed, 
or  in  which  was  being  laid  down  material  that  was  subsequently 
removed,  leaving  little  or  no  record  of  its  existence. 
SUMMARY    AND    DESCRIPTION    OF    FORMATIONS. 
The  following  table  is  a  brief  description  of  evidence  that  has 
been  collected  from  this  general  region,  but  includes  only  that  part 
of  the  stratigraphic  column  most  directly  related  to  a  discussion  of 
the  coal-bearing  rocks. 
Rock  formations  of  the  northwestern  Colorado  coalfields. 
Geo- 
logic 
age. 
Forma- 
tion. 
Description  of  si  rata. 
Topographic  features. 
Thickness. 
Economic  value. 
Probably  late  Tertiary  or 
younger. 
a 
o 
I 
o 
M 
a 
Pi 
tn 
% 
C 
r 
Consists  of  loose  or 
slightly       consoli- 
d  ited    sandy   ma- 
terial    with     local 
harder    sandstone 
beds  and  some  beds 
of  gravel.        Con- 
tains     much     cal- 
careous      material 
in  the  form  of  ce- 
menl  or  filling  be- 
tween   the  quartz 
sand  grains.       Its 
color  is  everywhere 
chalky      or      limv 
Forms  low  sand  hills 
or    valley    country, 
commonly  covered 
sparsely  with  sage- 
brush, greasewood, 
cedar,     or     pifion. 
Typically    exposed 
in       Axial       Basin 
about     Juniper 
Mountain,  and  sup- 
posed      t0      extend 
continuously  north- 
west \va  rd     into 
Browns  Park. 
Not  determined. 
