﻿CHAPTER 
  11. 
  

  

  LA 
  PLATA 
  VALLEY. 
  

  

  The 
  Rio 
  La 
  Plata, 
  although 
  a 
  small 
  stream, 
  is 
  one 
  of 
  the 
  most 
  impor- 
  

   tant 
  in 
  the 
  southwest. 
  It 
  takes 
  its 
  rise 
  in 
  the 
  rich 
  mining-district 
  of 
  the 
  

   La 
  Plata 
  Mountains, 
  and 
  in 
  its 
  middle 
  course 
  flows 
  through 
  considera- 
  

   ble 
  areas 
  of 
  tillable 
  and 
  pasture 
  lands. 
  Its 
  entire 
  length 
  is 
  60 
  miles. 
  

   For 
  8 
  miles 
  of 
  the 
  upper 
  course 
  it 
  is 
  in 
  a 
  deep 
  mountain-valley. 
  Below 
  

   Parrott 
  City 
  it 
  runs 
  some 
  4 
  miles 
  through 
  a 
  flat 
  valley 
  occupied 
  by 
  pla- 
  

   cer-bars. 
  On 
  leaving 
  this 
  it 
  cuts 
  its 
  way 
  through 
  a 
  ridge 
  of 
  Upper 
  Cre- 
  

   taceous 
  sandstone 
  and 
  enters 
  a 
  broad, 
  shallow 
  valley. 
  The 
  upper 
  part 
  of 
  

   the 
  latter 
  valley 
  is 
  occupied 
  by 
  a 
  fine 
  system 
  of 
  terraces. 
  Of 
  these 
  there 
  

   are 
  a 
  number 
  of 
  distinct 
  levels, 
  the 
  higher 
  occupying 
  the 
  divide 
  between 
  

   the 
  La 
  Plata 
  and 
  Lightner's 
  Fork 
  of 
  the 
  Animas. 
  In 
  the 
  vicinity 
  of 
  the 
  

   road-crossing 
  this 
  is 
  nearly 
  300 
  feet 
  above 
  the 
  bed 
  of 
  the 
  La 
  Plata 
  and 
  

   upward 
  of 
  1,200 
  above 
  the 
  bed 
  of 
  the 
  Animas. 
  West 
  of 
  the 
  river 
  they 
  

   are 
  not 
  so 
  well 
  marked. 
  

  

  Cherry 
  Creek 
  is 
  the 
  only 
  considerable 
  tributary 
  of 
  the 
  La 
  Plata, 
  and 
  

   gathers 
  the 
  drainage 
  from 
  the 
  southwestern 
  spurs 
  of 
  the 
  La 
  Plata 
  Mount- 
  

   ains. 
  It 
  has 
  its 
  eastern 
  source 
  within 
  a 
  mile 
  of 
  the 
  La 
  Plata 
  at 
  Parrott 
  

   City. 
  From 
  this 
  point 
  it. 
  runs 
  west 
  inside 
  or 
  north 
  of 
  the 
  northeastern 
  

   wing 
  of 
  the 
  Mesa 
  Verde. 
  From 
  Thompson's 
  Park 
  it 
  cuts 
  its 
  way 
  out 
  

   through 
  the 
  sandstone 
  wall 
  to 
  the 
  south 
  and 
  enters 
  the 
  broad 
  valley 
  of 
  

   the 
  La 
  Plata. 
  Itjoinsthelatter 
  stream 
  just 
  midway 
  in 
  its 
  course 
  and 
  in 
  

   the 
  center 
  of 
  the 
  valley. 
  The 
  conformation 
  of 
  this 
  broad 
  valley 
  is 
  the 
  

   result 
  of 
  what 
  would 
  at 
  first 
  appear 
  to 
  be 
  quite 
  unimportant 
  geologic 
  con- 
  

   ditions. 
  It 
  occupies 
  the 
  space 
  between 
  two 
  slight 
  monoclinal 
  folds. 
  One 
  

   of 
  these 
  extends 
  west 
  from 
  the 
  La 
  Plata 
  Mountains 
  and 
  dies 
  out 
  in 
  the 
  

   Mesa 
  Verde; 
  the 
  other 
  crosses 
  the 
  country 
  nearly 
  20 
  miles 
  farther 
  south, 
  

   and 
  sweeps 
  off 
  to 
  the 
  southwest. 
  Along 
  the 
  northern 
  border 
  the 
  former 
  

   fold 
  has 
  upturned 
  the 
  Upper 
  Cretaceous 
  beds, 
  producing 
  a 
  rim 
  of 
  sand- 
  

   stones. 
  The 
  dip 
  of 
  these 
  sandstones 
  is 
  at 
  first 
  from 
  5° 
  to 
  10°, 
  but 
  as 
  we 
  

   pass 
  south 
  through 
  the 
  valley 
  we 
  find 
  that 
  they 
  flatten 
  out 
  and 
  descend 
  

   almost 
  with 
  the 
  fall 
  of 
  the 
  river. 
  On 
  reaching 
  the 
  second 
  fold 
  a 
  gentle 
  

   dropping 
  occurs 
  on 
  the 
  south 
  or, 
  more 
  properly, 
  southeast, 
  by 
  which 
  the 
  

   Bad 
  Land 
  Group 
  (2, 
  general 
  section) 
  is 
  brought 
  to 
  the 
  river-level, 
  and 
  

   the 
  sandstones 
  of 
  the 
  Upper 
  Coal 
  Group 
  (3, 
  general 
  section) 
  form 
  a 
  low 
  

   hogback 
  facing 
  the 
  north, 
  which 
  gives 
  a 
  low 
  southern 
  rim 
  to 
  the 
  valley. 
  

   Slight 
  as 
  the 
  dip 
  seems 
  to 
  be 
  on 
  the 
  floor 
  of 
  this 
  valley 
  between 
  the 
  folds, 
  

   a 
  considerable 
  thickness 
  of 
  beds, 
  consisting 
  of 
  the 
  upper 
  strata 
  of 
  the 
  

   Mesa 
  Verde 
  Group, 
  disappears, 
  although 
  few 
  outcrops 
  are 
  seen. 
  

  

  The 
  second 
  or 
  outer 
  fold 
  just 
  mentioned 
  is 
  one 
  of 
  the 
  most 
  important 
  

   features 
  in 
  the 
  geology 
  of 
  the 
  ISau 
  Juan 
  Valley. 
  I 
  shall 
  therefore 
  call 
  it 
  

   the 
  Sau 
  Juan 
  fold. 
  It 
  seems 
  to 
  begin 
  along 
  the 
  base 
  of 
  the 
  mountains 
  

   east 
  of 
  the 
  Animas 
  River, 
  and 
  is 
  first 
  noticeable 
  in 
  the 
  increased 
  dip 
  it 
  

   gives 
  to 
  the 
  outcropping 
  strata 
  of 
  the 
  Lower 
  Cretaceous. 
  This 
  increased 
  

   dip 
  was 
  noticed 
  by 
  Dr. 
  Endlich 
  in 
  1874. 
  My 
  attention 
  was 
  first 
  drawn 
  

  

  245 
  

  

  