﻿^20 
  Geological 
  Society. 
  

  

  those 
  of 
  ' 
  moels 
  ' 
  such 
  as 
  Moel 
  Tryfaen, 
  or 
  like 
  the 
  well-worn 
  

   Appalachian 
  Mountains 
  of 
  North 
  Carolina, 
  or 
  the 
  Cevennes. 
  

  

  The 
  chief 
  abnormal 
  features 
  of 
  Snowdon 
  are 
  the 
  following 
  : 
  — 
  

   Alongside 
  the 
  graded 
  summit 
  and 
  slopes 
  of 
  a 
  ' 
  moel 
  ' 
  stand 
  the 
  

   head-cliffs 
  of 
  a 
  rock- 
  walled 
  cwm, 
  in 
  the 
  floor 
  of 
  which 
  talus 
  is 
  now 
  

   accumulating. 
  The 
  cwm- 
  floors 
  are 
  generally 
  stepped, 
  sometimes 
  

   more 
  than 
  once, 
  and 
  the 
  streams 
  cascade 
  down 
  into 
  the 
  valleys. 
  

   The 
  cross-profile 
  of 
  the 
  valleys 
  is 
  often 
  a 
  fine 
  catenary 
  curve, 
  down 
  

   the 
  sides 
  of 
  which 
  streams 
  fall 
  from 
  hanging 
  valleys. 
  The 
  slope 
  

   of 
  the 
  main 
  valleys 
  occasionally 
  decreases 
  even 
  to 
  the 
  point 
  of 
  

   reversal, 
  as 
  where 
  lakes 
  occur 
  ; 
  and 
  in 
  the 
  immediate 
  neighbourhood 
  

   of 
  smoothly-graded, 
  waste-covered 
  slopes 
  knobby 
  or 
  craggy 
  ledges 
  

   and 
  bars 
  of 
  rock 
  often 
  appear. 
  After 
  pointing 
  out 
  that 
  such 
  

   features 
  are 
  generally 
  associated 
  with 
  glaciation, 
  the 
  author 
  pro- 
  

   ceeds 
  to 
  discuss 
  two 
  out 
  of 
  four 
  possible 
  hypotheses 
  put 
  forward 
  : 
  

   ' 
  that 
  glaciers 
  are 
  essentially 
  protective 
  agencies,' 
  or 
  that 
  they 
  ' 
  are 
  

   active 
  destructive 
  agencies.' 
  The 
  consequences 
  deduced 
  from 
  the 
  

   hypotheses 
  are 
  confronted 
  with 
  the 
  actual 
  facts, 
  and 
  it 
  is 
  found 
  

   that 
  these, 
  and 
  especially 
  those 
  relating 
  to 
  rock-steps, 
  cannot 
  be 
  

   explained 
  on 
  the 
  protection 
  -theory, 
  while 
  the 
  theory 
  of 
  a 
  destructive 
  

   agency 
  seems 
  to 
  explain 
  most 
  of 
  the 
  facts. 
  

  

  These 
  facts 
  are 
  dealt 
  with 
  under 
  the 
  following 
  heads 
  : 
  — 
  Valley- 
  

   head 
  curves 
  ; 
  valley-floors, 
  lakes 
  ; 
  valley-floors, 
  rock-steps 
  ; 
  valley- 
  

   sides 
  ; 
  hanging 
  lateral 
  valleys 
  ; 
  and 
  glacial 
  overflows. 
  With 
  regard 
  

   to 
  the 
  first, 
  it 
  is 
  shown 
  that 
  there 
  is 
  no 
  systematic 
  relationship 
  

   between 
  the 
  height 
  of 
  the 
  cwm-clifl's 
  and 
  the 
  distance 
  of 
  the 
  front 
  

   rock-step 
  ; 
  the 
  serration 
  of 
  ' 
  cribs 
  ' 
  or 
  aretes 
  cannot 
  be 
  explained 
  by 
  

   pre-Glacial 
  or 
  post-Glacial 
  weathering 
  according 
  to 
  the 
  protection- 
  

   theory. 
  It 
  is 
  suggested 
  that 
  it 
  might 
  be 
  possible 
  on 
  the 
  erosion- 
  

   theory 
  to 
  work 
  out 
  and 
  classify 
  cwms 
  according 
  to 
  their 
  age 
  and 
  

   growth, 
  and 
  as 
  a 
  contribution 
  to 
  this 
  enquiry 
  the 
  cwms 
  of 
  Mynydd 
  

   Mawr 
  are 
  dealt 
  with. 
  The 
  valley-lakes 
  are 
  likely 
  to 
  be 
  only 
  a 
  

   small 
  part 
  of 
  the 
  glacial 
  erosion. 
  No 
  consistent 
  explanation 
  of 
  the 
  

   valley-steps 
  can 
  be 
  found 
  under 
  the 
  theory 
  of 
  ice-protection, 
  

   whereas 
  they 
  are 
  explicable 
  on 
  the 
  assumption 
  of 
  glacial 
  erosion. 
  

   They 
  may 
  have 
  originated 
  in 
  resistant 
  beds, 
  and 
  have 
  then 
  retreated 
  

   up 
  the 
  valleys. 
  The 
  catenary 
  curve 
  of 
  the 
  cross-section 
  of 
  such 
  

   valleys 
  as 
  those 
  containing 
  Llyn 
  Gwynant 
  and 
  Llyn 
  Cwellin 
  might 
  

   be 
  expected 
  to 
  result 
  from 
  long-continued 
  ice-erosion 
  ; 
  and 
  the 
  

   occurrence 
  of 
  great 
  cliifs 
  on 
  the 
  sides 
  of 
  these 
  valleys 
  is 
  not 
  incon- 
  

   sistent 
  with 
  such 
  an 
  origin. 
  Several 
  good 
  examples 
  of 
  hanging 
  

   valleys 
  occur, 
  and 
  they 
  seem 
  to 
  show 
  that 
  the 
  deepening 
  of 
  the 
  

   main 
  valleys 
  by 
  glacial 
  erosion 
  may 
  be 
  from 
  200 
  to 
  400 
  feet, 
  and 
  in 
  

   some 
  cases 
  as 
  much 
  as 
  500 
  or 
  600 
  feet. 
  The 
  lateral 
  erosion 
  may 
  

   easily 
  amount 
  to 
  1000 
  or 
  1500 
  feet. 
  The 
  most 
  striking 
  case 
  of 
  a 
  

   glacial 
  overflow 
  is 
  that 
  at 
  the 
  head 
  of 
  the 
  Nantlle 
  valley, 
  which 
  

   appears 
  to 
  have 
  carried 
  much 
  of 
  the 
  West 
  Snowdon 
  ice. 
  The 
  head 
  

   of 
  the 
  pass 
  would 
  seem 
  to 
  have 
  been 
  farther 
  westward 
  and 
  higher 
  

   in 
  pre-Grlacial 
  times. 
  

  

  