﻿1864.] 
  MAW 
  SEVERX- 
  VALLEY 
  DRIFT. 
  143 
  

  

  bottom, 
  it 
  is 
  easy 
  to 
  understand 
  how 
  such 
  thick 
  masses 
  as 
  that 
  at 
  

   Strethill 
  might 
  accumulate 
  and 
  fill 
  up 
  the 
  narrow 
  valley. 
  

  

  When 
  we 
  reflect 
  on 
  the 
  variety 
  of 
  conditions 
  under 
  which 
  the 
  drift 
  

   must 
  have 
  been 
  deposited 
  and 
  modified, 
  from 
  the 
  time 
  of 
  the 
  first 
  

   erosion 
  of 
  the 
  older 
  formations 
  to 
  the 
  present 
  denuded 
  remnants 
  of 
  

   gravel, 
  we 
  need 
  not 
  be 
  surprised 
  at 
  the 
  variety 
  of 
  aspects 
  it 
  presents. 
  

   The 
  vast 
  changes 
  of 
  outline 
  of 
  the 
  older 
  rocks 
  involved 
  in 
  merely 
  

   providing 
  materials 
  for 
  its 
  formation, 
  and 
  the 
  continual 
  changes 
  of 
  

   outline 
  of 
  land 
  and 
  water 
  resulting 
  from 
  the 
  depression 
  and 
  subse- 
  

   quent 
  elevation 
  of 
  land, 
  of 
  at 
  least 
  900 
  feet, 
  would 
  repeatedly 
  vary 
  

   the 
  proportion 
  of 
  materials 
  brought 
  under 
  erosive 
  action. 
  

  

  When 
  it 
  is 
  remembered 
  that 
  these 
  changes 
  of 
  contour 
  in 
  an 
  island- 
  

   covered 
  sea, 
  full 
  of 
  narrow 
  and 
  changing 
  channels, 
  would 
  continually 
  

   vary 
  the 
  force 
  and 
  direction 
  of 
  the 
  eroding 
  currents, 
  that 
  such 
  

   changes 
  may 
  have 
  greatly 
  varied 
  in 
  neighbouring 
  localities, 
  and 
  that 
  

   these 
  deposits 
  of 
  debris 
  must, 
  in 
  the 
  subsidence 
  and 
  elevation 
  of 
  the 
  

   land, 
  have 
  been 
  often 
  torn 
  up 
  and 
  redepositedwith 
  different 
  proportions 
  

   of 
  freshly 
  eroded 
  matter, 
  we 
  shall 
  not 
  wonder 
  at 
  drifts 
  apparently 
  of 
  

   contemporary 
  deposition 
  presenting 
  in 
  different 
  localities 
  that 
  variety 
  

   of 
  aspect 
  which 
  renders 
  their 
  comparison 
  so 
  perplexing, 
  and 
  is 
  so 
  

   notably 
  different 
  from 
  the 
  persistent 
  character 
  of 
  deep-sea 
  deposits. 
  

  

  Drift 
  may 
  occur, 
  whether 
  on 
  the 
  present 
  surface 
  of 
  the 
  earth 
  or 
  

   amongst 
  the 
  older 
  geological 
  formations, 
  wherever 
  denudation 
  has 
  

   taken 
  place, 
  and 
  must 
  essentially 
  be 
  local 
  in 
  character 
  and 
  irregular 
  

   in 
  distribution 
  ; 
  it 
  is 
  often 
  dissevered 
  from 
  the 
  finer 
  sedimentary 
  de- 
  

   posits 
  contemporaneously 
  derived 
  from 
  the 
  degradation 
  of 
  antecedent 
  

   formations, 
  and 
  probably 
  carried 
  hundreds 
  of 
  miles 
  away 
  into 
  the 
  

   deepest 
  basins 
  of 
  the 
  ocean. 
  

  

  Drift-matter 
  generally 
  implies 
  the 
  existence 
  of 
  an 
  old 
  coast-line. 
  

   It 
  was 
  seldom 
  accumulated 
  except 
  near 
  the 
  tidal 
  range 
  ; 
  and 
  in 
  

   tracing 
  it 
  from 
  low 
  to 
  high 
  ground, 
  we 
  must 
  attribute 
  its 
  wide 
  dis- 
  

   tribution 
  and 
  range 
  of 
  altitude 
  to 
  the 
  progressive 
  advance 
  or 
  retro- 
  

   gression 
  of 
  the 
  waters, 
  making 
  successively 
  every 
  part 
  of 
  the 
  land 
  

   on 
  which 
  it 
  was 
  formed 
  an 
  old 
  sea-border, 
  and 
  bringing 
  it 
  under 
  the 
  

   erosive 
  action 
  of 
  the 
  waves. 
  

  

  We 
  may 
  suppose 
  the 
  gradual 
  advance 
  of 
  the 
  waters, 
  first 
  forming- 
  

   estuaries 
  in 
  our 
  lowest 
  valleys, 
  then 
  creeping 
  up 
  the 
  hill-sides, 
  

   severing 
  England 
  from 
  Wales, 
  and 
  England 
  into 
  two 
  or 
  three 
  large 
  

   islands, 
  and 
  then 
  forming 
  the 
  mountain-tops 
  into 
  an 
  archipelago, 
  de- 
  

   positing 
  a 
  succession 
  of 
  beaches 
  at 
  various 
  levels 
  during 
  its 
  progress 
  ; 
  

   and 
  possibly 
  the 
  whole 
  land 
  was 
  covered 
  by 
  a 
  widely 
  expanding 
  

   ocean 
  unmarked 
  by 
  a 
  single 
  mountain 
  -top 
  or 
  islet. 
  

  

  From 
  the 
  very 
  large 
  proportion 
  of 
  the 
  debris 
  that 
  has 
  accumulated 
  

   in 
  our 
  valleys, 
  compared 
  with 
  that 
  on 
  the 
  high 
  ground, 
  we 
  are 
  too 
  

   apt 
  to 
  picture 
  to 
  ourselves 
  that 
  the 
  valleys 
  were 
  almost 
  exclusively 
  

   the 
  scene 
  of 
  the 
  submergence 
  that 
  the 
  drift 
  represents. 
  But 
  the 
  

   lower 
  the 
  level 
  of 
  the 
  old 
  coast-line, 
  the 
  larger 
  was 
  its 
  circuit, 
  and 
  

   the 
  greater 
  would 
  be 
  the 
  bulk 
  of 
  eroded 
  matter 
  ; 
  and 
  in 
  looking 
  for 
  

   evidence 
  of 
  the 
  presence 
  of 
  the 
  waters 
  at 
  high 
  levels, 
  we 
  must 
  not 
  

   expect 
  to 
  find 
  the 
  same 
  bulk 
  of 
  drift 
  as 
  in 
  the 
  lowlands, 
  on 
  account 
  

  

  