286         CONTRIBUTIONS   TO   ECONOMIC   GEOLOGY,   190*7,   PART   II. 
Rock  formations  of  the  northwestern  Colorado  coalfields — Continued. 
Geo- 
logic 
age. 
Forma- 
tion. 
Description  of  strata.    Topographic  features. 
Thickness. 
Economic  value. 
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Composed   of   shale, 
sandstone, and  beds 
of  oolitic  rock.  The 
stialy     beds     pre- 
dominate and  are 
very  compact  and 
firmly      bedded. 
They  are  generally 
exposed  in  escarp- 
ments    and     high 
bluffs  in  which  the 
weathered    beds 
have  a  very  char- 
acteristic    chalky- 
white  aspect.    The 
shales    are,    how- 
ever,    of     various 
shades     of     gray, 
drab,     and     light 
brown  and  are  in 
many  places  hard 
and    thin   bedded. 
In   some   districts 
the  lower  part  con- 
tains   much    mas- 
sive   white    sand- 
stone. 
Characterized  by  pla- 
teaus   where    it    is 
flat  lying;  these  re- 
gions    being     bor- 
dered by  high  and 
abrupt    escarp- 
ments at  the  mar- 
gins or  near  the  up- 
tilted  areas.      This 
fomiation    is    only 
slightly  affected  by 
weathering    except 
where  broken  or  ex- 
posed   in    a    tilted 
position.    It  is  cut 
by    deep    canyons 
along    the    stream 
channels,  as  it  read- 
ily gives  way  to  ero- 
sion by   the  attri- 
tion of  mechanically 
transported  harder 
material.     Jt  is  well 
represented  in  the 
Roan  or  Book  Cliffs 
Plateau,  the  Cathe- 
dral Bluffs,  and  the 
Gray  Hills. 
Measured  sections 
exceed  2, 400feet; 
upper  limit  not 
reached. 
Gilsonite  and   re- 
.  lated    hydrocar- 
bons in  the  Uin- 
ta Basin. 
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Composed  chiefly  of 
clay   or   soft   clay 
shale;      commonly 
variegated,      but 
various   shades  of 
red  and  drab  pre- 
dominate.    It  also 
contains    beds    of 
pebbles     or     con- 
glomerate of  very 
perfectly    rounded 
silicious    material, 
such  as  jasper,  col- 
ored vein   quartz, 
chert,      or      flint. 
Sandstones     in 
places    very    mas- 
sive and  hard. 
Commonly   weathers 
to    low    valley    or 
ridge    country,    at 
some  places  scarred 
by  badland  washes. 
It    is   usually  dis- 
tinguished  by   the 
banded  or  colored 
clays  in  the  bluffs 
underlying   or    ad- 
jacent to  the  escarp- 
ments of  the  Green 
River      formation. 
Cactus    Valley,  on 
Grand     River,     is 
eroded  from  these 
beds. 
From  4,000  feet  on 
the  eastern  side 
of  the  Uinta 
Basin  to  about 
2,500  feet  near 
the  Utah  line. 
Oil  and  asphaltum 
deposits  near 
Vernal,  Utah. 
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Beds  not  readily  dis- 
tinguished     'from 
the"    Wasatch     in 
the    Uinta    Basin, 
but        apparently 
more  clearly  differ- 
entiated    in      the 
Yampa  field.      As 
developed    in    the 
latter  region  they 
consist  of  massive 
white      or      light- 
colored  sandstones 
and     shales     con- 
taining     valuable, 
coal   beds   sharply 
defined  at  the  base 
by  a  conglomerate 
or     conglomeratic 
sandstone,      with- 
out doubt  marking 
an  unconformity  of 
considerable  mag- 
nitude. 
Forms     ridges     and 
hilly    country,    es- 
pecially where  the 
sandstone  members 
are  prominent  and 
numerous.    Topog- 
raphy       resembles 
that   of   the   other 
coal-bearing       for- 
mations,   although 
not    commonly    so 
rugged  as   that  of 
the  Alesaverde. 
Estimated    as 
about   800    feet 
on    Fay   Creek 
north  of  Lay. 
Workable  coal. 
Unconformity. 
