﻿218 
  REPORT 
  UNITED 
  STATES 
  GEOLOGICAL 
  SURVEY. 
  

  

  and 
  partly 
  volcauic, 
  erratic 
  material 
  that 
  was 
  brought 
  down 
  by 
  the 
  

   movement 
  of 
  the 
  ice. 
  

  

  On 
  either 
  side 
  of 
  the 
  valley 
  are 
  Cretaceous 
  ridges, 
  which 
  close 
  at 
  its 
  

   end 
  to 
  a 
  narrow 
  caiion. 
  At 
  one 
  time 
  evidently 
  this 
  caiion 
  was 
  choked 
  

   up 
  with 
  the 
  accumulating 
  drift, 
  and 
  the 
  ice 
  found 
  itself 
  forced 
  to 
  expand 
  

   laterally. 
  Thereby 
  a 
  large 
  mass 
  of 
  the 
  bowlders 
  was 
  pushed 
  against 
  

   the 
  low 
  divide 
  eastward, 
  through 
  which 
  formerly 
  the 
  next 
  stream 
  to 
  

   the 
  east 
  entered 
  and 
  joined 
  the 
  ice-field. 
  At 
  the 
  same 
  time 
  this 
  eastern 
  

   fork 
  (Eio 
  Huerto), 
  situated 
  between 
  stations 
  36 
  and 
  37, 
  was 
  but 
  a 
  glacial 
  

   stream, 
  resulting 
  from 
  the 
  melted 
  ice 
  that 
  filled 
  the 
  valley 
  between 
  the 
  

   two 
  stations. 
  

  

  To-day 
  that 
  valley 
  consists 
  of 
  a 
  beautiful 
  green 
  meadow 
  four 
  miles 
  

   long 
  and 
  about 
  a 
  mile 
  wide. 
  It 
  is 
  flat, 
  with 
  a 
  pretty 
  stream 
  running 
  

   along 
  its 
  eastern 
  edge. 
  Along 
  both 
  the 
  sides 
  morainal 
  benches 
  follow 
  

   its 
  entire 
  length, 
  and 
  at 
  the 
  lower 
  end 
  of 
  the 
  valley 
  a 
  terminal 
  moraine 
  

   has 
  constructed 
  a 
  ''dam" 
  about 
  60 
  feet 
  high. 
  Near 
  the 
  center 
  of 
  this 
  

   dam 
  is 
  an 
  opening 
  of 
  regular 
  shape 
  through 
  which 
  the 
  stream 
  finds 
  its 
  

   outlet. 
  It 
  turns 
  sharply 
  to 
  the 
  east, 
  runs 
  in 
  a 
  southerly 
  direction 
  around 
  

   a 
  Cretaceous 
  bluff 
  and 
  joins 
  the 
  stream 
  of 
  Weeminuche 
  Valley 
  about 
  

   six 
  miles 
  lower 
  down. 
  The 
  glacier 
  coming 
  out 
  of 
  the 
  narrow 
  gorge 
  of 
  

   volcanic 
  mountains 
  at 
  the 
  head 
  of 
  the 
  valley 
  moved 
  down 
  it, 
  depositing 
  

   more 
  erratic 
  material 
  on 
  the 
  west 
  than 
  on 
  the 
  east 
  side, 
  owing 
  to 
  the 
  

   slope 
  of 
  the 
  latter. 
  At 
  th^t 
  time 
  the 
  waters 
  flowing 
  off 
  from 
  it 
  joined 
  

   the 
  western 
  creek 
  five 
  miles 
  higher 
  up 
  than 
  at 
  present. 
  Gradually, 
  how- 
  

   ever, 
  the 
  passage 
  became 
  obstructed 
  by 
  the 
  deposition 
  of 
  moraines, 
  and 
  

   the 
  water 
  could 
  no 
  longer 
  flow 
  through 
  the 
  accustomed 
  channel. 
  Dur- 
  

   ing 
  the 
  same 
  time 
  the 
  eastern 
  glacier 
  pushed 
  its 
  own 
  terminal 
  moraine 
  

   toward 
  the 
  narrow 
  southern 
  entrance 
  of 
  the 
  valley, 
  until 
  it 
  was 
  no 
  longer 
  

   possible 
  for 
  the 
  water 
  to 
  escape 
  there, 
  save 
  in 
  disconnected, 
  small 
  

   brooks. 
  Either 
  synchronous 
  with 
  this 
  period 
  or 
  shortly 
  after, 
  the 
  gla- 
  

   ciers 
  of 
  both 
  valleys 
  began 
  to 
  recede, 
  first, 
  perhaps, 
  the 
  eastern 
  one, 
  

   and 
  then 
  the 
  accumulating 
  waters 
  of 
  the 
  Huerto 
  Valley 
  formed 
  a 
  lake. 
  

   From 
  the 
  present 
  appearance 
  of 
  the 
  moraines, 
  from 
  reduction 
  of 
  their 
  

   prominent 
  irregular 
  shapes 
  to 
  that 
  of 
  continuous 
  benches, 
  and 
  from 
  the 
  

   gradual 
  but 
  extensive 
  erosion 
  produced 
  by 
  overflowing 
  waters, 
  I 
  have 
  

   concluded 
  that 
  the 
  lake 
  remained 
  there 
  for 
  quite 
  a 
  long 
  time. 
  Finally, 
  

   however, 
  the 
  resisting 
  terminal 
  moraines 
  seem 
  to 
  have 
  been 
  weakened 
  

   and 
  the 
  water 
  rushed 
  forth, 
  breaking 
  the 
  opening 
  into 
  the 
  dam 
  that 
  

   now 
  exists. 
  Instead 
  of 
  being 
  able 
  to 
  flow 
  in 
  the 
  same 
  southwesterly 
  

   direction 
  that 
  it 
  followed 
  before 
  the 
  deposition 
  of 
  moraines 
  at 
  the 
  point 
  of 
  

   its 
  egress, 
  it 
  was 
  forced 
  thereby 
  to 
  turn 
  eastward, 
  and 
  only 
  after 
  flowing 
  

   for 
  six 
  miles 
  could 
  it 
  find 
  an 
  opening 
  through 
  which 
  to 
  join 
  the 
  stream 
  

   that 
  it 
  formerly 
  entered 
  higher 
  up. 
  These 
  two 
  instances 
  are 
  the 
  most 
  

   indisputable 
  that 
  I 
  have 
  seen 
  in 
  Southern 
  Colorado 
  of 
  the 
  former 
  exist- 
  

   ence 
  of 
  glacial 
  lakes. 
  Where 
  by 
  the 
  recessionof 
  the 
  glaciers 
  throughout 
  

   the 
  region 
  was 
  produced, 
  will 
  be 
  discussed 
  in 
  subsequent 
  pages. 
  How 
  

   slowly 
  or 
  rapidly 
  it 
  may 
  have 
  occurred, 
  however, 
  and 
  how 
  much 
  oppor- 
  

   tunity 
  for 
  the 
  formation 
  of 
  lakes 
  may 
  thereby 
  have 
  been 
  afforded, 
  is 
  a 
  

   question 
  that 
  cannot 
  be 
  answered 
  by 
  the 
  observations 
  and 
  experience 
  of 
  

   to-day. 
  

  

  It 
  seems 
  probable 
  that 
  a 
  number 
  of 
  the 
  cailons 
  through 
  which 
  tribu- 
  

   taries 
  of 
  the 
  Eios 
  Piedra 
  and 
  San 
  Juan 
  flow 
  were 
  cut 
  in 
  part 
  by 
  glaciers. 
  

   The 
  large 
  amount 
  of 
  drift 
  deposition 
  in 
  the 
  immediate 
  vicinity 
  of 
  the 
  

   mountains, 
  which 
  there 
  form 
  a 
  long 
  line 
  of 
  steep 
  slopes, 
  cannot 
  well 
  be 
  

   otherwise 
  accounted 
  for. 
  In 
  case 
  we 
  could 
  assume 
  that 
  the 
  mean 
  annual 
  

   temperature 
  in 
  that 
  region 
  was 
  at 
  any 
  time 
  low 
  enough 
  to 
  admit 
  of 
  the 
  

  

  