﻿THE 
  GLACIAL 
  PERIOD 
  IN 
  NORTH 
  BRITAIN. 
  337 
  

  

  return 
  of 
  a 
  great 
  ice-sheet 
  which 
  spread 
  over 
  nearly 
  the 
  whole 
  of 
  

   Scotland 
  and 
  Ireland 
  and 
  also 
  the 
  greater 
  part 
  of 
  England. 
  But 
  

   the 
  ice 
  seems 
  not 
  to 
  have 
  been 
  so 
  thick 
  and 
  extensive 
  as 
  it 
  was 
  in 
  

   the 
  early 
  glaciation 
  — 
  nor 
  so 
  enduring 
  ; 
  for 
  it 
  has 
  failed 
  to 
  destroy 
  all 
  

   the 
  beds 
  of 
  clay 
  and 
  sand 
  containing 
  arctic 
  shells 
  which 
  the 
  sea 
  left 
  

   behind 
  it, 
  whereas 
  in 
  Scotland 
  we 
  find 
  that 
  the 
  ice 
  of 
  the 
  former 
  

   period 
  cleaned 
  off 
  every 
  thing 
  down 
  to 
  the 
  hard 
  rock. 
  I 
  would 
  sup- 
  

   pose 
  that 
  all 
  the 
  mountain-ranges 
  of 
  Scotland 
  and 
  Wales 
  were 
  coated 
  

   with 
  thick 
  ice, 
  which 
  reached 
  the 
  coast 
  in 
  most 
  places, 
  likewise 
  the 
  

   hilly 
  ground 
  of 
  the 
  north 
  of 
  England, 
  including 
  the 
  Pennine 
  ridge 
  

   along 
  its 
  whole 
  length 
  as 
  far 
  at 
  least 
  as 
  Derby. 
  In 
  the 
  lower 
  dis- 
  

   tricts 
  of 
  England 
  further 
  south, 
  there 
  would 
  probably 
  be 
  extensive 
  

   snow-beds, 
  more 
  or 
  less 
  converted 
  into 
  ice 
  at 
  the 
  bottom. 
  During 
  

   the 
  summer 
  thaws 
  these 
  would 
  send 
  out 
  great 
  streams 
  of 
  muddy 
  

   water, 
  occasioning 
  those 
  superficial 
  deposits 
  of 
  brick-earth, 
  warp, 
  and 
  

   loess 
  which 
  are 
  so 
  widely 
  spread 
  to 
  the 
  southward. 
  Beds 
  of 
  gravel 
  

   would 
  be 
  lodged 
  where 
  the 
  force 
  of 
  the 
  current 
  was 
  stronger, 
  and 
  

   when 
  the 
  thaw 
  was 
  unusually 
  rapid. 
  That 
  there 
  must 
  have 
  been 
  

   heavy 
  beds 
  of 
  consolidated 
  snow 
  even 
  in 
  the 
  extreme 
  south 
  of 
  

   England, 
  I 
  infer 
  from 
  the 
  fact 
  that 
  I 
  find 
  traces 
  of 
  such 
  in 
  my 
  own 
  

   neighbourhood, 
  in 
  the 
  low 
  eastern 
  part 
  of 
  Aberdeenshire, 
  on 
  the 
  

   flanks 
  of 
  hills 
  only 
  a 
  few 
  hundred 
  feet 
  in 
  height. 
  In 
  the 
  low 
  ground 
  

   in 
  front 
  of 
  these 
  places 
  there 
  is 
  generally 
  a 
  land-locked 
  hollow 
  or 
  

   shallow 
  basin 
  occupied 
  by 
  peat. 
  

  

  This 
  second 
  ice-sheet 
  gradually 
  shrank 
  into 
  smaller 
  and 
  smaller 
  

   compass, 
  separating 
  by 
  degrees 
  into 
  valley-glaciers, 
  which 
  paused 
  for 
  

   a 
  time 
  here 
  and 
  there 
  in 
  their 
  retreat, 
  leaving 
  Kaims 
  and 
  Eskers 
  in 
  

   the 
  low 
  grounds 
  and 
  more 
  distinct 
  moraines 
  in 
  the 
  mountain-glens. 
  

   At 
  times 
  the 
  ice 
  seems 
  to 
  have 
  receded 
  rapidly 
  over 
  a 
  wide 
  area 
  

   without 
  leaving 
  any 
  marginal 
  deposits. 
  In 
  these 
  places 
  the 
  surface 
  

   is 
  more 
  or 
  less 
  covered 
  with 
  a 
  sheet 
  of 
  coarse 
  earth 
  or 
  mud 
  mixed 
  

   with 
  stones, 
  which 
  formed 
  the 
  bed 
  or 
  lair 
  of 
  the 
  ice. 
  During 
  such 
  

   times 
  there 
  probably 
  occurred 
  a 
  succession 
  of 
  warmer 
  seasons, 
  causing 
  

   a 
  considerable 
  thaw, 
  when 
  vast 
  bodies 
  of 
  water 
  would 
  flow 
  down 
  the 
  

   valleys, 
  spreading 
  out 
  much 
  gravel, 
  sand, 
  and 
  silt 
  along 
  their 
  course. 
  

   But 
  it 
  is 
  important 
  to 
  observe 
  that 
  no 
  great 
  submergence 
  of 
  the 
  

   country 
  again 
  occurred, 
  nothing 
  but 
  that 
  slight 
  depression 
  which 
  is 
  

   marked 
  by 
  the 
  estuary 
  beds 
  and 
  raised 
  beaches 
  a 
  little 
  above 
  the 
  

   present 
  coast-bine. 
  

  

  The 
  last 
  great 
  modification 
  of 
  the 
  surface 
  has 
  been 
  subaerial, 
  and 
  

   not 
  submarine. 
  Glaciers, 
  frost, 
  and 
  rain 
  have 
  done 
  the 
  work, 
  not 
  

   the 
  waves 
  of 
  the 
  sea. 
  

  

  Discussion. 
  

  

  Mr. 
  Jeffreys 
  considered 
  that 
  the 
  author's 
  remarks 
  relating 
  to 
  the 
  

   beds 
  containing 
  Arctic 
  species 
  of 
  MoJlusca 
  were 
  not 
  quite 
  correct. 
  

   Pecten 
  islandicus 
  has 
  been 
  found 
  in 
  the 
  drift 
  of 
  Scotland, 
  but 
  not 
  in 
  

   the 
  seas 
  at 
  present 
  surrounding 
  that 
  country. 
  At 
  depths 
  of 
  30 
  or 
  

   40 
  fathoms 
  many 
  Arctic 
  shells 
  in 
  a 
  semifossil 
  state 
  have 
  been 
  

   dredged, 
  although 
  they 
  do 
  not 
  now 
  live 
  in 
  those 
  waters. 
  Mya 
  trim- 
  

  

  