fenneman.]  BOULDER    OIL    FIELD,   COLORADO.  385 
the  position  of  the  beds.  Such  indications  and  the  still  rather  indefi- 
nite inferences  from  the  comparison  of  well  sections  are  the  only 
direct  means  by  which  structure  can  be  determined  in  the  immediate 
vicinity  of  the  oil  wells. 
It  is  otherwise  with  the  borders  of  the  area.  On  the  west  are  the 
fine  exposures  of  the  firm  rocks  of  the  foothills.  Their  clearly 
revealed  structure  is  now  seen  to  show  peculiarities  corresponding  to 
the  essential  structural  features  of  the  oil  field.  In  the  shales  of  the 
oil  field  these  features  can  be  made  out  only  by  wearisome  search; 
their  correlatives  in  the  foothills  are  open  to  the  light  of  day.  The 
practical  significance  of  this  relation  will  be  evident  to  any  who  may 
be  prospecting  for  similar  deposits  of  oil  along  the  front  of  the  range. 
On  the  south,  east,  and  north  are  also  good  exposures,  though  only 
those  of  the  north  have  structural  significance  comparing  in  impor- 
tance with  the  western  outcrops. 
Folds. — The  general  dip  of  the  Fort  Pierre  shales  and  lower  strata 
is  a  little  south  of  east.  There  is  at  the  same  time  a  tendency  to  fold- 
ing along  axes  running  north  and  south  or  northwest  and  southeast. 
Strong  folding  of  this  character  causes  actual  anticlines  whose  direc- 
tion is  not  quite  parallel  to  the  foothills,  and  the  folds  therefore  run 
out  in  the  foothill  upturns.  The  folding  may  not  be  so  strong  and 
then  there  is  no  actual  anticline,  but  only  a  check  in  the  general  east- 
ward dip.  When  an  actual  anticline  occurs  the  effect  on  an  out- 
cropping stratum  is  to  cause  its  edge  to  make  a  long  loop,  nearly 
parallel  with  the  general  trend  of  its  outcrop.  When  the  folding  does 
not  succeed  in  making  a  true  anticline  but  causes  only  an  arrest  in  the 
prevailing  dip  the  effect  on  its  outcrop  is  to  cause  a  jog,  the  northern 
part  (in  this  case)  being  offset  to  the  east  of  the  southern  portion. 
Both  these  conditions  are  clearly  illustrated  in  the  accompanying 
sketch  map.  Such  structure  is  called  echelon  folding.  It  is  the  char- 
acteristic kind  for  a  long  distance  north  of  Boulder. 
It  has  been  pointed  out  before  that  all  the  wells  which  nave  thus  far 
produced  oil  are  along  a  narrow  north-south  belt.  It  is  now  known 
that  this  line  follows  a  monoclinal  fold  of  the  Fort  Pierre  shales,  the 
wells  being  near  its  top  and  therefore  in  similar  position  to  those  of 
Ohio,  described  by  Orton  as  occuring  on  " arrested  anticlines."  If 
extended  northward,  this  monocline  is  almost  in  line  with  an  incipient 
echelon  fold,  which  affects  the  outcrop  of  the  sandstone  seen  in  sec. 
6,  by  causing  a  strong  offset  to  the  east.  The  axis  of  this  fold  points 
south-southeast.  About  5  miles  farther  northeast  a  more  strongly 
developed  fold  of  similar  character  is  revealed  by  the  outcrop  of  the 
same  sandstone.  The  fold  runs  almost  due  south  so  far  that  an  impor- 
tant part  of  it  lies  east  of  the  smaller  one  mentioned  above.  Both 
these  folds  are  described  below.  The  monocline,  which  determines  the 
present  line  of  producing  wells,  is  the  southward  continuation  of  the 
Bull.  225—04 25  - 
