﻿142 
  PROCEEDINGS 
  OF 
  THE 
  GEOLOGICAL 
  SOCIETY. 
  [Jail. 
  20, 
  

  

  during 
  the 
  deposition 
  of 
  the 
  drift. 
  The 
  more 
  we 
  look 
  into 
  the 
  drift- 
  

   phenomena, 
  the 
  more 
  convinced 
  are 
  we 
  how 
  very 
  different 
  the 
  contour 
  

   of 
  the 
  country 
  must 
  have 
  been 
  before 
  the 
  submergence 
  of 
  the 
  land, 
  

   notwithstanding 
  the 
  probable 
  existence 
  of 
  the 
  present 
  larger 
  river- 
  

   valleys, 
  and 
  we 
  should 
  hesitate 
  before 
  assigning 
  exact 
  distances 
  for 
  

   the 
  transit 
  of 
  rocks 
  which 
  now 
  only 
  occur 
  in 
  situ 
  a 
  long 
  way 
  off. 
  

   Two 
  separate 
  epochs 
  of 
  denudation 
  appear 
  evident, 
  — 
  firstly, 
  that 
  of 
  

   the 
  denudation 
  of 
  the 
  older 
  formations 
  to 
  supply 
  the 
  materials 
  of 
  the 
  

   drift, 
  and 
  secondly, 
  after 
  an 
  intervening 
  period 
  of 
  comparative 
  rest, 
  

   that 
  of 
  the 
  erosion 
  of 
  the 
  drift 
  itself, 
  accompanied 
  by 
  a 
  further 
  de- 
  

   gradation 
  of 
  the 
  older 
  rocks 
  ; 
  for 
  the 
  rounded 
  contour 
  of 
  the 
  knolls 
  

   of 
  drift 
  occurring 
  at 
  Strethill 
  and 
  Buildwas, 
  as 
  before 
  observed, 
  is 
  

   quite 
  independent 
  of 
  the 
  stratification 
  of 
  their 
  gravels, 
  and, 
  with 
  

   the 
  rounded 
  contour 
  of 
  the 
  Wenlock 
  shale 
  and 
  other 
  older 
  forma- 
  

   tions, 
  appears 
  as 
  though 
  the 
  whole 
  had 
  been 
  eroded 
  together. 
  

  

  I 
  would 
  suggest 
  that 
  the 
  first 
  degradation 
  of 
  the 
  older 
  formations 
  

   took 
  place 
  during 
  the 
  submergence 
  of 
  the 
  land, 
  that 
  the 
  drift- 
  deposits 
  

   had 
  attained 
  their 
  greatest 
  thickness 
  when 
  the 
  full 
  submergence 
  was 
  

   accomplished, 
  and 
  that, 
  during 
  the 
  gradual 
  elevation 
  of 
  the 
  land, 
  when 
  

   its 
  whole 
  surface 
  would 
  have 
  successively 
  again 
  come 
  under 
  erosive 
  

   action, 
  the 
  drift 
  was 
  gradually 
  denuded 
  with 
  a 
  further 
  degradation 
  

   of 
  the 
  older 
  formations. 
  

  

  It 
  is 
  impossible 
  to 
  compare 
  the 
  numerous 
  outlying 
  patches 
  of 
  

   drift, 
  the 
  various 
  levels 
  at 
  which 
  they 
  occur, 
  and 
  the 
  great 
  individual 
  

   thickness 
  of 
  some 
  of 
  the 
  isolated 
  patches, 
  without 
  being 
  convinced 
  of 
  

   the 
  large 
  proportion 
  that 
  has 
  been 
  removed, 
  compared 
  with 
  what 
  

   remains. 
  

  

  The 
  valley 
  above 
  Ironbridge 
  is 
  shut 
  in 
  by 
  hills 
  rising 
  500 
  

   feet 
  above 
  the 
  present 
  level 
  of 
  the 
  Severn 
  ; 
  and 
  as 
  it 
  now 
  contains 
  

   knolls 
  of 
  drift, 
  upwards 
  of 
  200 
  feet 
  high, 
  that 
  have 
  escaped 
  the 
  ero- 
  

   sive 
  action 
  of 
  the 
  subsiding 
  waters, 
  it 
  appears 
  more 
  than 
  probable 
  

   that 
  the 
  valley 
  up 
  to 
  and 
  beyond 
  Shrewsbury 
  was 
  at 
  one 
  time 
  (before 
  

   the 
  second 
  erosion) 
  filled 
  up 
  with 
  a 
  great 
  and 
  even 
  mass 
  of 
  the 
  drift- 
  

   deposit. 
  The 
  Strethill 
  knoll 
  was 
  evidently 
  not 
  an 
  isolated 
  patch, 
  

   for 
  at 
  its 
  very 
  summit, 
  200 
  feet 
  above 
  the 
  river, 
  it 
  has 
  a 
  level 
  stra- 
  

   tification 
  quite 
  independent 
  of 
  its 
  rounded 
  outline, 
  and 
  must 
  have 
  

   extended 
  across 
  the 
  valley, 
  and 
  formed 
  a 
  barrier 
  adjacent 
  to 
  the 
  

   gorge, 
  between 
  the 
  Silurian 
  ridge 
  of 
  Benthall 
  Edge 
  on 
  the 
  west 
  and 
  

   Lincoln 
  Hill 
  on 
  the 
  east, 
  through 
  which 
  the 
  Severn 
  slowly 
  ate 
  out 
  

   its 
  course 
  during 
  the 
  elevation 
  of 
  the 
  land. 
  It 
  is 
  scarcely 
  possible 
  

   to 
  speculate 
  as 
  to 
  the 
  original 
  consecutive 
  thickness 
  of 
  this 
  deposit 
  ; 
  

   but 
  I 
  may 
  mention 
  that 
  at 
  Byden 
  Hill, 
  on 
  the 
  top 
  of 
  Benthall 
  Edge 
  

   (see 
  fig. 
  1), 
  not 
  above 
  a 
  mile 
  and 
  a 
  half 
  from 
  the 
  Strethill 
  knoll, 
  a 
  

   deposit 
  of 
  drift- 
  sand 
  and 
  shingle 
  occurs, 
  250 
  feet 
  above 
  the 
  stratified 
  

   summit 
  of 
  Strethill, 
  with 
  which 
  it 
  is 
  precisely 
  identical 
  in 
  character. 
  

  

  That 
  such 
  a 
  vast 
  consecutive 
  thickness 
  as 
  nearly 
  500 
  feet 
  once 
  

   existed 
  of 
  a 
  deposit 
  which 
  now 
  has 
  but 
  a 
  local 
  distribution 
  seems 
  

   scarcely 
  probable 
  ; 
  but 
  as 
  such 
  isolated 
  patches, 
  many 
  of 
  them 
  of 
  

   considerable 
  bulk, 
  occur 
  on 
  the 
  high 
  land 
  immediately 
  adjacent 
  to 
  

   the 
  valley, 
  at 
  elevations 
  of 
  from 
  400 
  to 
  550 
  feet 
  above 
  its 
  present 
  

  

  