﻿EXBLICH,] 
  

  

  APPE^^DIX 
  ANCIENT 
  GLACIEES 
  221 
  

  

  uudoiibted 
  glacial 
  regions 
  of 
  Southern 
  Colorado 
  tbat 
  I 
  have 
  had 
  occa- 
  

   sion 
  to 
  visit 
  during 
  the 
  years 
  1874 
  and 
  1875. 
  An 
  interesting 
  question 
  

   presents 
  itself 
  when 
  considering 
  whether 
  these 
  glacial 
  occurrences 
  were 
  

   synchronous 
  or 
  not, 
  firstly, 
  among 
  themselves, 
  and 
  secondly, 
  with 
  the 
  

   accepted 
  glacial 
  epoch 
  of 
  this 
  continent. 
  Inasmuch 
  as 
  the 
  latter 
  ex- 
  

   tended 
  over 
  a 
  very 
  large 
  period 
  of 
  time, 
  there 
  is 
  but 
  little 
  to 
  be 
  gained 
  

   or 
  lost 
  by 
  discussing 
  that 
  portion 
  of 
  the 
  question. 
  The 
  first 
  part 
  of 
  it, 
  

   however, 
  has 
  a 
  direct 
  bearing 
  upon 
  the 
  physical 
  history 
  of 
  Colorado, 
  

   and 
  it 
  will 
  be 
  well 
  to 
  examine 
  into 
  the 
  facts 
  that 
  may 
  lead 
  to 
  the 
  one 
  or 
  

   other 
  view. 
  Undoubtedly 
  the 
  most 
  valuable 
  evidence 
  in 
  determining 
  

   the 
  geological 
  age 
  of 
  this 
  glacier 
  is 
  to 
  be 
  gained 
  by 
  observing 
  upon 
  

   what 
  rocks 
  or 
  beds 
  they 
  were 
  located. 
  But 
  little 
  will 
  be 
  learned 
  on 
  

   this 
  point 
  from 
  that 
  at 
  the 
  head 
  of 
  the 
  Animas. 
  During 
  the 
  Carbonifer- 
  

   ous 
  period, 
  probably, 
  or 
  late 
  during 
  the 
  Devonian, 
  the 
  rocks 
  that 
  now 
  

   exist 
  as 
  quartzites, 
  schists, 
  and 
  granites, 
  were 
  metamorphosed 
  so 
  as 
  to 
  

   attain 
  their 
  present 
  character. 
  Those 
  near 
  station 
  38 
  of 
  1874 
  are 
  of 
  Silu- 
  

   rian 
  age, 
  so 
  that 
  nothing 
  regarding 
  younger 
  formations 
  can 
  be 
  learned 
  

   at 
  that 
  locality. 
  On 
  the 
  Piedra 
  the 
  glaciers 
  have 
  covered 
  Lower 
  Creta- 
  

   ceous 
  sandstones 
  and 
  shales; 
  but 
  as 
  this 
  is 
  the 
  youngest 
  group 
  in 
  ex- 
  

   istence 
  there, 
  the 
  age 
  is 
  too 
  remote 
  for 
  us 
  to 
  take 
  the 
  relative 
  position 
  

   in 
  evidence. 
  The 
  Conejos 
  glacier, 
  however, 
  affords 
  more 
  of 
  a 
  clew. 
  A 
  

   spur 
  of 
  the 
  plateau 
  from 
  which 
  it 
  started 
  runs 
  in 
  a 
  southeasterly 
  direc- 
  

   tion, 
  and 
  upon 
  this 
  station 
  88 
  is 
  located 
  at 
  an 
  elevation 
  of 
  12,181 
  feet. 
  

   This 
  is 
  lower 
  than 
  station 
  85, 
  12,282 
  feet, 
  which 
  latter 
  station 
  is 
  still 
  

   within 
  the 
  original 
  domain 
  of 
  the 
  glacier. 
  In 
  spite 
  of 
  this 
  difference 
  in 
  

   altitude 
  the 
  plateau 
  of 
  88 
  is 
  covered 
  by 
  a 
  black 
  vesicular 
  basalt, 
  while 
  

   85 
  is 
  located 
  on 
  trachyte. 
  Experience 
  and 
  observation 
  have 
  taught 
  

   that 
  basalt 
  (as 
  a 
  rule, 
  though 
  exceptions 
  in 
  favor 
  of 
  rhyolite 
  are 
  claimed) 
  

   is 
  the 
  youngest 
  of 
  the 
  volcanic 
  rocks. 
  Both 
  trachyte 
  and 
  basalt 
  are 
  re- 
  

   garded 
  as 
  being 
  of 
  Tertiary 
  origin, 
  extending 
  in 
  time 
  to 
  the 
  Miocene 
  

   period. 
  Here, 
  then, 
  we 
  have 
  a 
  tangible 
  fact, 
  whereby 
  to 
  determine 
  the 
  

   relative 
  age 
  of 
  at 
  least 
  one 
  glacier. 
  A 
  portion 
  of 
  the 
  Conejos 
  glacier 
  

   extended 
  toward 
  the 
  southeast 
  and 
  covered 
  a 
  portion 
  of 
  the 
  basalt 
  

   which 
  there 
  overlies 
  the 
  trachyte. 
  It 
  is 
  certain, 
  then, 
  that 
  the 
  period 
  of 
  

   existence 
  for 
  that 
  glacier 
  must 
  have 
  fallen 
  into 
  late 
  Tertiary 
  times, 
  pro- 
  

   vided 
  onr 
  premise 
  as 
  to 
  the 
  age 
  of 
  trachyte 
  and 
  basalt 
  is 
  correct. 
  How 
  

   much 
  later 
  than 
  the 
  period 
  stated 
  it 
  may 
  have 
  existed 
  there, 
  however, 
  is 
  

   beyond 
  the 
  possibility 
  of 
  determination. 
  We 
  have 
  merely 
  the 
  extreme 
  

   limit 
  of 
  " 
  old 
  age." 
  

  

  Taking 
  into 
  consideration 
  the 
  accumulation 
  of 
  finely 
  separated 
  

   mineral 
  matter 
  in 
  San 
  Luis 
  Yalley, 
  which 
  may 
  in 
  part 
  owe 
  its 
  existence 
  

   to 
  the 
  action 
  of 
  the 
  glaciers 
  as 
  stated 
  above, 
  we 
  see 
  between 
  the 
  two 
  

   items 
  a 
  connection 
  as 
  to 
  time. 
  The 
  glacier 
  covered 
  basalt 
  and 
  the 
  

   detritus 
  produced 
  by 
  the 
  glacier 
  covers 
  iDasalt.* 
  Having 
  seen 
  this 
  to 
  

   be 
  the 
  case, 
  we 
  have 
  at 
  hand 
  the 
  means 
  to 
  explain 
  the 
  afflux 
  of 
  an 
  un- 
  

   usual 
  amount 
  of 
  water 
  into. 
  San 
  Luis 
  Valley, 
  which 
  at 
  that 
  time 
  must 
  

   have 
  been 
  in 
  a 
  condition 
  resembling 
  the 
  present 
  in 
  its 
  general 
  features. 
  

   Ifthe 
  basalt 
  was 
  there 
  the 
  bluffs 
  and 
  mountains 
  in 
  the 
  south 
  of 
  the 
  

   valley 
  were 
  already 
  formed, 
  and 
  the 
  gentle 
  slope 
  from 
  the 
  southeast 
  

   was 
  also 
  existing. 
  That 
  the 
  glacier 
  did 
  disappear 
  is 
  a 
  fact, 
  also 
  that 
  it 
  

   could 
  not 
  have 
  vanished 
  solely 
  by 
  the 
  agency 
  of 
  evaporation. 
  Melting 
  

   of 
  the 
  ice 
  would 
  of 
  necessity 
  accompany 
  any 
  process 
  of 
  that 
  nature, 
  and 
  

   the 
  supply 
  of 
  water 
  for 
  San 
  Luis 
  Valley 
  would 
  have 
  been 
  greatly 
  in- 
  

   creased. 
  It 
  is 
  possible 
  to 
  answer 
  the 
  question 
  of 
  contemporaneous 
  

  

  * 
  Report 
  ou 
  Northern 
  California 
  and 
  Oregon, 
  J. 
  S. 
  Newberry, 
  1657, 
  p. 
  42, 
  mentions 
  

   " 
  trap-ledges 
  " 
  grooved 
  and 
  striated 
  by 
  glaciers. 
  

  

  