﻿480 
  X. 
  G. 
  BONNEY 
  ON 
  THE 
  GLACIER-EROSION 
  

  

  Thus 
  I 
  have 
  said 
  but 
  little 
  about 
  direction 
  of 
  striae, 
  &c, 
  because, 
  

   as 
  a 
  rule, 
  the 
  bolder 
  configuration 
  of 
  the 
  terrain 
  was 
  sufficient, 
  with- 
  

   out 
  the 
  confirmatory 
  evidence 
  of 
  minor 
  marking, 
  to 
  indicate 
  to 
  an 
  

   educated 
  eye 
  the 
  path 
  of 
  the 
  ice-streams, 
  — 
  and 
  little 
  about 
  the 
  stra- 
  

   tigraphy 
  of 
  the 
  country 
  (although 
  that 
  was 
  often 
  carefully 
  noted), 
  

   because, 
  as 
  a 
  rule, 
  owing 
  to 
  its 
  many 
  disturbances, 
  the 
  evidence 
  

   furnished 
  by 
  it 
  was 
  neither 
  favourable 
  nor 
  unfavourable 
  to 
  Professor 
  

   Ramsay's 
  theory 
  or 
  to 
  my 
  own. 
  

  

  The 
  observations 
  in 
  this 
  communication 
  concern 
  principally 
  (1) 
  

   the 
  district 
  of 
  the 
  Upper 
  Engadine 
  and 
  of 
  the 
  Yal 
  Bregaglia 
  with 
  

   the 
  Lake 
  of 
  Como, 
  and 
  (2) 
  the 
  Italian 
  valleys 
  of 
  Monte 
  Rosa 
  

   including 
  the 
  Lake 
  of 
  Orta. 
  I 
  had 
  already 
  travelled 
  more 
  than 
  

   once 
  over 
  the 
  greater 
  part 
  of 
  these 
  regions, 
  together 
  with 
  that 
  of 
  

   the 
  other 
  Italian 
  lakes 
  ; 
  but, 
  during 
  the 
  summer 
  of 
  1873, 
  1 
  devoted 
  

   myself 
  to 
  a 
  more 
  minute 
  reexamination 
  of 
  these, 
  because, 
  from 
  previ- 
  

   ous 
  experience, 
  I 
  knew 
  them 
  to 
  be 
  among 
  the 
  most 
  important. 
  

  

  I 
  assume 
  throughout 
  this 
  communication 
  : 
  — 
  

  

  1. 
  That 
  gorges 
  and 
  Y-shaped 
  valleys 
  are 
  mainly 
  cut 
  by 
  " 
  rain 
  and 
  

   rivers." 
  

  

  2. 
  That 
  valleys 
  or 
  basins 
  excavated 
  or 
  materially 
  modified 
  by 
  

   glaciers 
  must 
  have 
  a 
  trough-like 
  form, 
  that 
  of 
  a 
  broad 
  U. 
  

  

  3. 
  That, 
  except 
  in 
  those 
  cases 
  where 
  there 
  is 
  a 
  sudden 
  change 
  

   from 
  a 
  hard 
  to 
  a 
  soft 
  rock, 
  the 
  contours 
  of 
  a 
  valley, 
  whose 
  form 
  has 
  

   been 
  much 
  modified 
  by 
  a 
  glacier, 
  would 
  remain 
  approximately 
  

   uniform 
  — 
  viz. 
  that 
  if 
  a 
  lake-basin 
  has 
  been 
  cut 
  by 
  a 
  glacier, 
  the 
  

   valley 
  above 
  must 
  also 
  be 
  more 
  or 
  less 
  U-shaped. 
  

  

  I 
  commence 
  with 
  the 
  Engadine 
  district, 
  where, 
  unfortunately, 
  

   owing 
  to 
  the 
  unusual 
  lateness 
  of 
  the 
  summer, 
  my 
  results 
  in 
  one 
  or 
  

   two 
  cases 
  were 
  less 
  complete 
  than 
  I 
  had 
  hoped 
  to 
  make 
  them. 
  

  

  In 
  the 
  Upper 
  Engadine 
  are 
  several 
  small 
  lakes 
  — 
  one 
  group 
  on 
  

   the 
  Bernina 
  Pass, 
  the 
  other 
  in 
  that 
  singular 
  trough 
  which 
  forms 
  the 
  

   head 
  of 
  the 
  Inn, 
  and 
  has 
  for 
  many 
  centuries 
  been 
  one 
  of 
  the 
  great 
  

   highways 
  of 
  Europe 
  across 
  the 
  Alps, 
  that 
  of 
  the 
  Maloya 
  Pass. 
  

  

  The 
  former 
  of 
  these 
  consists 
  of 
  two 
  lakes, 
  the 
  Lago 
  Bianco 
  and 
  the 
  

   Lago 
  Nero, 
  with 
  a 
  third 
  unimportant 
  tarn, 
  situated 
  at 
  a 
  height 
  of 
  

   about 
  7300' 
  above 
  the 
  sea, 
  nearly 
  on 
  the 
  summit 
  of 
  the 
  Bernina 
  

   Pass 
  (7658'). 
  The 
  Lago 
  Bianco 
  is 
  about 
  1| 
  mile 
  long 
  and 
  | 
  mile 
  

   broad 
  ; 
  the 
  Lago 
  Nero 
  is 
  very 
  much 
  smaller, 
  some 
  300 
  yards 
  long. 
  

   The 
  former 
  discharges 
  its 
  waters 
  towards 
  the 
  Adda, 
  the 
  latter 
  towards 
  

   the 
  Inn. 
  

  

  The 
  configuration 
  of 
  the 
  ground 
  in 
  which 
  these 
  lakes 
  lie 
  is 
  very 
  

   remarkable. 
  It 
  is 
  a 
  rather 
  broad 
  upland 
  trough, 
  about 
  two 
  miles 
  

   and 
  a 
  quarter 
  long, 
  running 
  NW. 
  and 
  SE., 
  dominated 
  on 
  the 
  

   south-western 
  side 
  by 
  the 
  steep 
  precipitous 
  buttresses 
  of 
  the 
  Piz 
  

   Cambrena, 
  the 
  summit 
  of 
  which 
  is 
  11,835' 
  above 
  the 
  sea, 
  on 
  the 
  

   other 
  by 
  rocky 
  hills 
  rising 
  a 
  few 
  hundred 
  feet 
  above 
  the 
  lakes 
  ; 
  the 
  

   south-eastern 
  end 
  broadens 
  out 
  into 
  a 
  hummocky 
  plateau, 
  no 
  part 
  of 
  

   which 
  is 
  more 
  than 
  about 
  a 
  hundred 
  yards 
  above 
  the 
  water. 
  The 
  

   torrent 
  from 
  the 
  Lago 
  Bianco 
  passes 
  over 
  some 
  marshy 
  ground, 
  once 
  

   part 
  of 
  the 
  lake, 
  and 
  descends 
  the 
  steep 
  head 
  of 
  the 
  Yal 
  di 
  Pila, 
  

  

  