﻿180 
  

  

  EEPOET 
  UNITED 
  STATES 
  GEOLOGICAL 
  SURVEY. 
  

  

  the 
  drainage, 
  and 
  are 
  sufficiently 
  cliaracteristic 
  afc 
  the 
  same 
  time 
  to 
  

   connect 
  regions 
  of 
  the 
  same 
  orographic 
  features. 
  

  

  A 
  synopsis 
  of 
  the 
  approximate 
  lengths 
  of 
  rivers 
  and 
  their 
  tributaries 
  

   occurring 
  in 
  the 
  region 
  to 
  which 
  this 
  chapter 
  is 
  devoted, 
  may 
  give 
  some 
  

   idea 
  of 
  the 
  relative 
  areas 
  they 
  drain. 
  Only 
  the 
  main 
  branches 
  of 
  the 
  

   tributaries 
  of 
  the 
  San 
  Juan 
  are 
  taken, 
  because 
  it 
  is 
  they 
  that 
  determine 
  

   the 
  horizontal 
  extent 
  of 
  the 
  drainage, 
  and 
  not 
  the 
  small 
  creeks 
  flowing 
  

   directly 
  into 
  the 
  main 
  stream. 
  The 
  northern 
  and 
  northeastern 
  portion 
  

   of 
  the 
  section 
  is 
  well 
  watered 
  ; 
  but 
  as 
  soon 
  as 
  the 
  Post-Cretaceous 
  beds 
  

   are 
  reached, 
  a 
  change 
  takes 
  place, 
  owing 
  mainly 
  to 
  the 
  lithological 
  

   character 
  of 
  the 
  strata 
  composing 
  the 
  bluffs 
  and 
  mesas. 
  

  

  Main 
  tributaries. 
  

  

  

  p 
  

  

  x> 
  

  

  a 
  

  

  

  

  ^ 
  

  

  d 
  

  

  

  

  ■♦^•d 
  

  

  a 
  

  

  

  > 
  

  

  .2^ 
  

  

  o 
  ® 
  

  

  Name 
  of 
  river. 
  

  

  ^i 
  

   ="3 
  

  

  a=s 
  

  

  

  

  °a 
  

  

  0.2 
  

  

  sa 
  

  

  

  ^ 
  

  

  M 
  

  

  til 
  

  

  -a 
  

  

  

  

  

  

  

  o 
  

  

  © 
  

  

  o 
  

  

  

  iJ 
  

  

  h^ 
  

  

  H 
  

  

  

  115 
  

  

  52 
  

   58 
  

   32 
  

  

  48 
  

  

  157 
  

   28 
  

   92 
  

   16 
  

   23 
  

  

  272 
  

  

  

  80 
  

  

  

  150 
  

  

  

  48 
  

  

  

  71 
  

  

  

  

  Branches 
  of 
  main 
  tributaries. 
  Lenjjth 
  

  

  in 
  miles. 
  

  

  Animas.. 
  Mineral 
  Creek 
  .., 
  - 
  16 
  

  

  Cascade 
  Creek 
  18 
  

  

  Kio 
  Arenosa 
  30 
  

  

  Junction 
  Creek 
  17 
  

  

  Eio 
  Florida 
  56 
  

  

  Pinos 
  .. 
  . 
  Eio 
  Vallecito 
  28 
  

  

  Piedra 
  .. 
  Weeminuche 
  Creek 
  22 
  

  

  Rio 
  Huerto 
  21 
  

  

  Dead 
  Man's 
  Creek 
  19 
  

  

  Eio 
  Nutria 
  30 
  

  

  Navajo 
  . 
  . 
  South 
  Branch 
  23 
  

  

  The 
  length 
  of 
  Eio 
  San 
  Juan 
  to 
  its 
  junction 
  with 
  the 
  Animas 
  is 
  about 
  

   one 
  hundred 
  and 
  forty 
  miles, 
  and 
  the 
  sum 
  of 
  its 
  entire 
  main 
  drainage 
  

   amounts 
  to 
  621 
  miles. 
  

  

  CRETACEOUS. 
  

  

  Cretaceous 
  beds 
  cover 
  a 
  considerable 
  area 
  in 
  this 
  section 
  of 
  the 
  dis- 
  

   trict. 
  Resting 
  against 
  the 
  base 
  of 
  the 
  mountains 
  toward 
  the 
  north 
  

   and 
  northeast, 
  we 
  find 
  the 
  Dakota 
  group. 
  It 
  is 
  strongly 
  developed 
  

   south 
  of 
  Weeminuche 
  Creek 
  but 
  soon 
  disappears, 
  being 
  covered 
  at 
  

   many 
  localities 
  by 
  the 
  younger 
  trachorheitic 
  beds. 
  In 
  some 
  of 
  the 
  

   canons 
  that 
  cut 
  deeply 
  into 
  the 
  volcanic 
  mass 
  the 
  characteristic 
  sand- 
  

   stones 
  crop 
  out, 
  but 
  upon 
  reaching 
  the 
  base 
  proper 
  of 
  the 
  mountains 
  we 
  

   find 
  them 
  covered 
  by 
  the 
  shales 
  of 
  the 
  Colorado 
  group 
  (Cretaceous 
  No. 
  2). 
  

   This 
  latter 
  comprises 
  a 
  by 
  far 
  greater 
  area 
  than 
  the 
  former, 
  extending 
  

   for 
  a 
  long 
  distance 
  from 
  east 
  to 
  west. 
  Many 
  of 
  the 
  streams 
  heading 
  in 
  

   the 
  Sawatch 
  Eange 
  flow 
  through 
  its 
  strata, 
  cutting 
  deep, 
  narrow 
  gorges 
  

   and 
  ravines, 
  or 
  producing 
  valleys 
  of 
  but 
  limited 
  breadth. 
  In 
  discussing 
  

   the 
  group, 
  it 
  will 
  be 
  observed 
  that 
  its 
  main 
  features 
  are 
  very 
  closely 
  

  

  