﻿THE 
  GLACIAL 
  PERIOD 
  IN 
  NORTH 
  BRITAIN. 
  319 
  

  

  most 
  likely 
  to 
  have 
  escaped 
  the 
  action 
  of 
  glaciers 
  ; 
  and 
  at 
  the 
  mouth 
  

   of 
  some 
  valleys 
  (as, 
  for 
  example, 
  that 
  of 
  the 
  Dee 
  at 
  Aberdeen) 
  we 
  

   find 
  masses 
  of 
  it 
  which 
  seem 
  to 
  be 
  the 
  denuded 
  remains 
  of 
  beds 
  that 
  

   some 
  powerful 
  agency 
  has 
  swept 
  clean 
  out 
  of 
  all 
  the 
  rest 
  of 
  the 
  

   valley. 
  And 
  some 
  of 
  these 
  beds 
  appear 
  to 
  have 
  been 
  dislodged 
  from 
  

   their 
  original 
  position 
  and 
  thrust 
  out 
  seawards 
  in 
  a 
  confused 
  mass, 
  

   as 
  in 
  the 
  banks 
  near 
  the 
  Aberdeen 
  lighthouse 
  and 
  powder-magazine 
  

   — 
  a 
  result 
  such 
  as 
  we 
  might 
  suppose 
  would 
  follow 
  from 
  a 
  glacier 
  

   moving 
  down 
  the 
  valley 
  over 
  the 
  beds 
  of 
  clay 
  and 
  sand, 
  wasting 
  

   them 
  gradually 
  beneath 
  it, 
  and 
  forcing 
  them 
  partly 
  before 
  it. 
  

  

  Now 
  let 
  us 
  consider 
  what 
  would 
  be 
  the 
  effect 
  of 
  a 
  glacier 
  extend- 
  

   ing 
  itself 
  over 
  beds 
  of 
  sand 
  and 
  clay 
  such 
  as 
  had 
  been 
  deposited 
  

   during 
  the 
  submergence 
  of 
  the 
  country. 
  The 
  advance 
  of 
  the 
  ice 
  

   over 
  this 
  old 
  sea-bottom 
  would 
  either 
  destroy 
  the 
  marine 
  beds, 
  by 
  

   pushing 
  them 
  before 
  it 
  and 
  wasting 
  them 
  beneath 
  it 
  (the 
  water 
  

   flowing 
  from 
  the 
  end 
  of 
  the 
  glacier 
  contributing 
  to 
  the 
  effect), 
  or 
  it 
  

   would 
  move 
  over 
  them 
  without 
  entirely 
  destroying 
  them. 
  Much 
  

   would 
  depend 
  upon 
  tbe 
  weight 
  of 
  ice, 
  its 
  duration, 
  the 
  slope 
  of 
  the 
  

   country, 
  and 
  also 
  the 
  depth 
  and 
  firmness 
  of 
  the 
  marine 
  strata. 
  

  

  Where 
  the 
  glacier 
  was 
  thick 
  and 
  the 
  valley 
  narrow 
  and 
  steep, 
  

   the 
  marine 
  beds 
  would 
  probably 
  be 
  entirely 
  cleared 
  out. 
  The 
  glacier 
  

   would 
  push 
  them 
  bodily 
  before 
  it 
  ; 
  and 
  where 
  it 
  did 
  slide 
  over 
  them, 
  

   the 
  pressure 
  of 
  the 
  ice 
  would 
  work 
  up 
  the 
  clay 
  and 
  sand 
  into 
  a 
  

   liquid 
  mud, 
  which 
  would 
  be 
  continually 
  carried 
  off 
  by 
  the 
  water 
  

   escaping 
  from 
  beneath 
  the 
  ice; 
  so 
  that 
  the 
  wasting 
  of 
  the 
  mass 
  

   would 
  go 
  on 
  rapidly, 
  and 
  result 
  in 
  its 
  entire 
  destruction, 
  leaving 
  

   only 
  the 
  stones 
  and 
  washed 
  gravel 
  in 
  front 
  of 
  the 
  glacier. 
  But 
  

   where 
  the 
  glacier 
  was 
  not 
  very 
  thick, 
  and 
  the 
  marine 
  strata 
  of 
  con- 
  

   siderable 
  depth 
  and 
  firmness, 
  I 
  imagine 
  the 
  ice 
  might 
  slide 
  over 
  such 
  

   a 
  deposit 
  without 
  entirely 
  destroying 
  it. 
  The 
  effect 
  would 
  probably 
  

   be 
  to 
  compress 
  and 
  wrinkle 
  it 
  to 
  some 
  degree, 
  and 
  to 
  work 
  the 
  upper 
  

   portion 
  of 
  it 
  into 
  an 
  unstratified 
  paste 
  or 
  mud 
  mixed 
  with 
  stones. 
  

   This 
  would 
  result 
  from 
  the 
  grinding 
  pressure 
  of 
  the 
  ice 
  and 
  the 
  

   want 
  of 
  good 
  drainage 
  for 
  the 
  water 
  beneath 
  it 
  ; 
  for 
  unless 
  there 
  

   was 
  a 
  current 
  of 
  water 
  to 
  carry 
  off 
  the 
  mud, 
  the 
  material 
  would 
  

   remain 
  beneath 
  the 
  glacier, 
  although 
  a 
  part 
  would 
  no 
  doubt 
  be 
  con- 
  

   tinually 
  carried 
  forward 
  by 
  adhering 
  to 
  its 
  sole 
  and 
  travelling 
  on 
  

   along 
  with 
  it. 
  The 
  pressure 
  of 
  the 
  ice 
  pushing 
  into 
  beds 
  of 
  strati- 
  

   fied 
  clay 
  and 
  sand 
  would, 
  in 
  some 
  cases, 
  displace 
  them, 
  and 
  wrinkle 
  

   them 
  into 
  folds, 
  thus 
  giving 
  rise 
  to 
  beds 
  of 
  contorted 
  drift. 
  

  

  The 
  unstratified, 
  unfossiliferous 
  mass 
  of 
  pebbly 
  clay 
  which 
  occu- 
  

   pies 
  so 
  much 
  of 
  the 
  surface, 
  and 
  is 
  occasionally 
  seen 
  overlying 
  marine 
  

   beds 
  containing 
  arctic 
  shells, 
  may 
  have 
  been 
  formed 
  in 
  the 
  way 
  

   above 
  described. 
  It 
  occurs 
  in 
  many 
  parts 
  of 
  Scotland, 
  and 
  has 
  got 
  

   the 
  name 
  of 
  upper 
  drift 
  or 
  upper 
  boulder-clay. 
  Much 
  of 
  what 
  has 
  

   been 
  occasionally 
  described 
  as 
  the 
  upper 
  covering 
  of 
  gravel 
  and 
  

   boulders 
  I 
  believe 
  to 
  be 
  also 
  a 
  result 
  of 
  the 
  action 
  of 
  the 
  later 
  gla- 
  

   ciers. 
  When 
  these 
  upper 
  beds 
  consist 
  of 
  marine 
  deposits 
  ground 
  

   into 
  mud 
  by 
  the 
  action 
  of 
  the 
  ice, 
  it 
  is 
  evident 
  that 
  bits 
  of 
  broken 
  

   shells 
  may 
  occasionally 
  be 
  found 
  in 
  them. 
  

  

  