﻿376 
  W. 
  H. 
  Twenhofel 
  — 
  Granite 
  Bowlders 
  in 
  (f 
  ) 
  

  

  view 
  the 
  bowlders 
  may 
  be 
  considered 
  either 
  a 
  remnant 
  of 
  a 
  

   once 
  extensive 
  sheet 
  of 
  drift, 
  or 
  a 
  local 
  deposit 
  of 
  ice 
  floating 
  

   from 
  the 
  glacial 
  margin 
  with 
  the 
  main 
  body 
  of 
  the 
  drift 
  lying 
  

   a 
  little 
  farther 
  to 
  the 
  north. 
  

  

  If 
  the 
  bowlders 
  are 
  a 
  remnant 
  of 
  a 
  once 
  extensive 
  sheet 
  of 
  

   drift, 
  then 
  the 
  rest 
  of 
  it 
  has 
  been 
  altogether 
  removed 
  from 
  the 
  

   surrounding 
  country 
  without 
  leaving 
  a 
  single 
  trace 
  of 
  its 
  one 
  

   time 
  presence 
  and 
  all 
  marks 
  of 
  glacial 
  erosion 
  have 
  been 
  com- 
  

   pletely 
  effaced 
  so 
  that 
  the 
  valleys 
  mentioned 
  above 
  show 
  no 
  

   evidence 
  whatever 
  of 
  having 
  been 
  filled 
  and 
  crossed 
  by 
  glaciers. 
  

   If 
  the 
  bowlders 
  are 
  the 
  deposits 
  of 
  early 
  Pleistocene 
  or 
  Ter- 
  

   tiary 
  floating 
  ice, 
  the 
  problem 
  is 
  rendered 
  no 
  simpler, 
  since 
  the 
  

   drift-covered 
  and 
  glacier-eroded 
  area 
  lay 
  but 
  a 
  short 
  distance 
  

   to 
  the 
  north. 
  

  

  This 
  view 
  postulates 
  great 
  erosion, 
  which 
  took 
  place 
  in 
  such 
  

   a 
  way 
  so 
  as 
  not 
  to 
  destroy 
  the 
  Rose 
  bowlders, 
  but 
  at 
  the 
  same 
  

   time 
  altogether 
  swept 
  away 
  similar 
  deposits 
  from 
  neighboring 
  

   areas, 
  some 
  of 
  which 
  were 
  higher 
  and 
  others 
  apparently 
  lower 
  

   than 
  the 
  hill 
  at 
  Hose. 
  Such 
  preservation 
  calls 
  for 
  extremely 
  

   special 
  conditions 
  and 
  there 
  is 
  nothing 
  in 
  the 
  region 
  that 
  lends 
  

   itself 
  to 
  the 
  view 
  that 
  such 
  were 
  present. 
  It 
  would 
  appear 
  that 
  

   the 
  final 
  stages 
  of 
  removal 
  of 
  the 
  supposed 
  drift 
  sheet 
  would 
  

   be 
  marked 
  by 
  gravels 
  of 
  small 
  size 
  remaining 
  here 
  and 
  there 
  

   over 
  a 
  great 
  stretch 
  of 
  country 
  instead 
  of 
  a 
  well-preserved 
  

   deposit 
  in 
  one 
  place. 
  

  

  The 
  impossible, 
  however, 
  in 
  the 
  past 
  has 
  not 
  infrequently 
  

   been 
  proved 
  the 
  possible 
  and 
  it 
  may 
  be 
  that 
  the 
  Rose 
  bowlders 
  

   are 
  of 
  early 
  Pleistocene 
  or 
  Tertiary 
  age. 
  If 
  such 
  be 
  the 
  case, 
  

   the 
  only 
  early 
  Pleistocene 
  glacial 
  material 
  with 
  which 
  

   it 
  is 
  possible 
  to 
  compare 
  them 
  is 
  the 
  sub-Aftonian 
  drift 
  of 
  

   Iowa. 
  Beyer,* 
  in 
  his 
  description 
  of 
  the 
  Oelwein 
  section, 
  char- 
  

   acterized 
  the 
  sub-Aftonian 
  drift 
  sheet 
  as 
  " 
  a 
  massive 
  gray-blue 
  

   till 
  with 
  a 
  marked 
  greenish 
  tone 
  when 
  unoxidized. 
  The 
  upper 
  

   portion 
  contains 
  much 
  humus 
  and 
  gives 
  off 
  a 
  characteristic 
  

   marsh-like 
  odor 
  when 
  wet. 
  The 
  distinctive 
  characters 
  which 
  

   serve 
  to 
  distinguish 
  the 
  bowlder 
  clay 
  from 
  the 
  preceding 
  

   (Kansan) 
  are 
  its 
  color, 
  the 
  predominance 
  of 
  greenstone, 
  and 
  

   vein 
  quartz 
  pebbles 
  and 
  a 
  less 
  tendency 
  to 
  joint 
  on 
  exposure. 
  

   Granitic 
  pebbles 
  and 
  bowlders 
  are 
  almost 
  if 
  not 
  entirely 
  want- 
  

   ing." 
  Savage 
  who 
  studied 
  the 
  same 
  section, 
  in 
  his 
  description 
  

   says 
  substantially 
  the 
  same 
  things. 
  f 
  The 
  sub-Aftonian 
  drift 
  

   of 
  the 
  Grand 
  River 
  section 
  was 
  described 
  by 
  Bain 
  as 
  " 
  Boulder 
  

   clay 
  . 
  . 
  . 
  containing 
  mainly 
  small 
  pebbles, 
  predominantly 
  of 
  

   vein 
  quartz, 
  but 
  with 
  a 
  fair 
  proportion 
  of 
  granite 
  "J 
  while 
  

  

  * 
  Beyer, 
  Proc. 
  Iowa 
  Acad. 
  Sci., 
  vol. 
  iv. 
  pp. 
  58-62, 
  1897. 
  

   f 
  Savage, 
  Arm. 
  Rept. 
  Iowa 
  Geol. 
  Surv., 
  vol. 
  xv, 
  p. 
  522, 
  1904. 
  

   JBain, 
  Proc. 
  Iowa 
  Acad. 
  Sc, 
  vol. 
  v, 
  p. 
  97. 
  1898; 
  Am. 
  Geol., 
  vol. 
  xxi, 
  

   p. 
  255, 
  1898. 
  

  

  