﻿■ 
  

  

  148 
  REPORT 
  UNITED 
  STATES 
  GEOLOGICAL 
  SURVEY. 
  «■ 
  

  

  the 
  sand-drift 
  areas 
  at 
  the 
  very 
  localities 
  where 
  at 
  present 
  lakes 
  could 
  

   be 
  produced 
  argues 
  strongly 
  for 
  the 
  correctness 
  of 
  the 
  view 
  that 
  such 
  

   lakes 
  have 
  existed 
  there 
  in 
  former 
  tiraes. 
  At 
  tbe 
  time 
  of 
  the 
  existence 
  

   of 
  these 
  lakes 
  the 
  inflowing 
  streams 
  that 
  often 
  carry 
  considerable 
  quanti- 
  

   ties 
  of 
  water 
  lost 
  their 
  impetus 
  upon 
  reaching 
  the 
  placid 
  sheet,, 
  and 
  

   thus 
  were 
  unable 
  to 
  transport 
  for 
  any 
  distance 
  the 
  rocky 
  material 
  they 
  

   had 
  so 
  far 
  brought 
  with 
  them. 
  Instead 
  of 
  flowing 
  out 
  ot 
  San 
  Luis 
  

   Valley 
  through 
  the 
  caiion, 
  the 
  Eio 
  Grande 
  found 
  its 
  exit 
  about 
  eight 
  

   miles 
  farther 
  east, 
  through 
  the 
  opening 
  which 
  leads 
  down 
  to 
  the 
  Rio 
  

   Grande 
  Valley. 
  It 
  may 
  seem 
  curious 
  that 
  no 
  heavy 
  deposits 
  ot 
  alkali 
  

   or 
  old 
  " 
  shore-lines" 
  mark 
  the 
  presence 
  of 
  these 
  ancient 
  lakes. 
  If, 
  how- 
  

   ever, 
  the 
  assumption 
  that 
  the 
  Grande 
  found 
  a 
  sudden 
  egress 
  through 
  

   the 
  deep 
  fissure 
  produced 
  by 
  a 
  volcanic 
  earthquake 
  is 
  true, 
  there 
  is 
  no 
  

   reason 
  why 
  th6 
  waters 
  should 
  not 
  have 
  flown 
  off 
  by 
  far 
  too 
  rapidly 
  to 
  

   permit 
  of 
  the 
  formation 
  of 
  either. 
  Eio 
  Oulebra, 
  after 
  the 
  emersion 
  of 
  the 
  

   land, 
  followed 
  a 
  southwesterly 
  course, 
  and, 
  joining 
  with 
  the 
  Costilla 
  near 
  

   the 
  entrance 
  of 
  the 
  caiion, 
  flowed 
  into 
  the 
  iiio 
  Grande. 
  Subsequently 
  

   this 
  was 
  changed, 
  and 
  the 
  Culebra 
  now 
  flows 
  nearly 
  due 
  west, 
  entering 
  

   the 
  Kio 
  Grande 
  just 
  below 
  station 
  105, 
  about 
  14 
  miles 
  farther 
  north. 
  

   An 
  accompanying 
  map 
  will 
  demonstrate 
  the 
  former 
  condition 
  ot 
  ban 
  

   Luis 
  Valley 
  and 
  the 
  changes 
  that 
  have 
  there 
  taken 
  place 
  by 
  far 
  bet- 
  

   ter 
  than 
  could 
  be 
  done 
  bv 
  description 
  alone. 
  Were 
  it 
  possible 
  to 
  make 
  

   a 
  survey 
  during 
  which 
  all 
  the 
  contours 
  were 
  accurately 
  measured, 
  1 
  

   doubt 
  not 
  that 
  the 
  old 
  boundaries 
  of 
  the 
  lakes 
  could 
  be 
  by 
  far 
  better 
  

   determined 
  than 
  it 
  has 
  been 
  possible 
  to 
  do 
  at 
  present. 
  . 
  a 
  .u 
  

  

  Upon 
  the 
  consideration 
  of 
  the 
  Glacial 
  period 
  in 
  this 
  section 
  ot 
  bouth- 
  

   ern 
  Colorado 
  the 
  presence 
  of 
  these 
  lakes 
  has 
  a 
  direct 
  bearing. 
  In 
  case 
  

   they 
  still 
  existed 
  at 
  that 
  time, 
  and 
  there 
  is 
  no 
  reason 
  why 
  they 
  should 
  

   not, 
  the 
  increased 
  evaporation, 
  the 
  dependent, 
  greater 
  precipitation, 
  

   and 
  a 
  decreased 
  mean 
  annual 
  temperature, 
  would 
  all 
  act 
  as 
  favorable 
  

   agents 
  to 
  the 
  formation 
  and 
  perpetuation 
  of 
  fields 
  of 
  ice. 
  I 
  do 
  not 
  

   mean 
  to 
  say 
  that 
  this 
  particular 
  presence 
  of 
  lakes 
  was 
  the 
  sole 
  cause 
  

   of 
  the 
  existence 
  of 
  glaciers 
  not 
  very 
  distant, 
  but 
  I 
  do 
  mean 
  to 
  say 
  that 
  

   it 
  aided 
  their 
  growth 
  and 
  progress. 
  On 
  the 
  other 
  hand, 
  in 
  case 
  these 
  

   lakes 
  were 
  drained, 
  glaciers, 
  but 
  a 
  short 
  distance 
  off", 
  would 
  have 
  lost 
  

   a 
  considerable 
  supply 
  of 
  moisture, 
  and 
  probably 
  the 
  mean 
  annual 
  

   temperature 
  of 
  immediate 
  surroundings 
  would 
  have 
  increased 
  percep- 
  

   tibly. 
  Certain 
  it 
  is 
  that 
  the 
  time 
  of 
  the 
  existence 
  and 
  disappearance 
  ot 
  

   the 
  San 
  Luis 
  Valley 
  lakes 
  must 
  fall 
  mainly 
  into 
  the 
  Post-Basaltic 
  period, 
  

   and 
  I 
  think 
  that 
  naming 
  a 
  time 
  near 
  the 
  close 
  of 
  the 
  glacial 
  epoch 
  in 
  

   Southern 
  Colorado 
  will 
  be 
  very 
  near 
  the 
  actual 
  time 
  of 
  the 
  formation 
  

   of 
  the 
  basaltic 
  fissure 
  through 
  which 
  the 
  two 
  lakes 
  were 
  drained. 
  In 
  

   Appendix 
  A, 
  the 
  former 
  glaciers 
  of 
  Southern 
  Colorado 
  and 
  the 
  time 
  of 
  

   their 
  existence 
  are 
  more 
  fully 
  discussed. 
  ^ 
  

  

  While 
  exploring 
  San 
  Luis 
  Valley 
  north 
  of 
  north 
  latitude 
  oi° 
  6{) 
  , 
  it 
  

   was 
  impossible 
  to 
  come 
  to 
  any 
  satisfactory 
  conclusions 
  with 
  regard 
  to 
  

   its 
  condition 
  in 
  former 
  geological 
  periods, 
  and 
  it 
  was 
  not 
  until 
  the 
  

   southeastern 
  corner 
  was 
  reached 
  that 
  the 
  sand 
  and 
  rocks 
  could 
  inter- 
  

   pret 
  their 
  meaning 
  intelligibly. 
  Analogous 
  cases 
  to 
  the 
  one 
  just 
  con- 
  

   sidered 
  we 
  have 
  in 
  Nevada 
  and 
  Southern 
  California, 
  besides 
  their 
  being 
  

   observed 
  in 
  other 
  countries 
  outside 
  of 
  our 
  own. 
  It 
  is 
  to 
  be 
  regretted 
  

   that 
  no 
  animal 
  remains 
  could 
  be 
  found 
  in 
  the 
  sand, 
  that 
  would 
  have 
  

   settled 
  the 
  question 
  more 
  positively, 
  but 
  it 
  is 
  highly 
  probable 
  that 
  the 
  

   lakes 
  were 
  alkaline, 
  although 
  fed 
  by 
  many 
  fresh 
  mountain-streams. 
  

   The 
  altitudes 
  given 
  on 
  the 
  map 
  will 
  convey 
  some 
  idea 
  of 
  the 
  uniformity 
  

   of 
  elevation 
  which 
  mav 
  be 
  observed 
  throughout 
  San 
  Luis 
  Valley. 
  

  

  