﻿MARINE 
  DRIFTS 
  IN 
  NORTH 
  WALES. 
  189 
  

  

  another 
  evidence 
  of 
  a 
  variation 
  in 
  the 
  rate 
  of 
  submergence, 
  the 
  

   rounded 
  stones 
  indicating 
  a 
  more 
  protracted 
  sojourD 
  of 
  the 
  sea 
  than 
  

   the 
  angular 
  stones 
  which, 
  during 
  a 
  rapid 
  subsidence 
  of 
  the 
  land, 
  

   would 
  soon 
  sink 
  beneath 
  the 
  rounding 
  action 
  of 
  waves 
  and 
  shallow 
  

   currents 
  (see 
  last 
  paper, 
  Q. 
  J, 
  Gr. 
  S. 
  for 
  August 
  1881, 
  p. 
  364*). 
  A 
  

   few 
  Eskdale 
  granite 
  pebbles 
  have 
  found 
  their 
  way 
  as 
  far 
  S. 
  as 
  

   Bryn 
  Adda 
  flat, 
  S.W. 
  of 
  which 
  I 
  searched 
  for 
  them 
  in 
  vain 
  f. 
  

  

  IY. 
  Probable 
  Origin 
  op 
  Drift-knolls. 
  

  

  Before 
  entering 
  on 
  this 
  subject 
  it 
  may 
  be 
  necessary 
  to 
  state- 
  that 
  

   the 
  drift-area 
  under 
  notice 
  is, 
  in 
  transverse 
  section, 
  a 
  slope 
  con- 
  

   sisting 
  of 
  an 
  alternation 
  of 
  flat 
  and 
  undulating 
  ground. 
  There 
  is 
  

   no 
  appearance 
  of 
  a 
  terraced 
  configuration 
  like 
  that 
  of 
  the 
  " 
  raised 
  

   beaches 
  " 
  of 
  the 
  W. 
  of 
  Scotland, 
  which 
  must 
  have 
  resulted 
  from 
  

   pauses 
  or 
  intervals 
  of 
  rest 
  between 
  successive 
  upward 
  movements 
  of 
  

   the 
  land 
  above 
  the 
  sea. 
  It 
  ought 
  likewise 
  to 
  be 
  stated 
  that 
  it 
  is 
  

   difficult, 
  if 
  not 
  impossible, 
  to 
  make 
  out 
  the 
  precise 
  order 
  of 
  succes- 
  

   sion 
  of 
  the 
  deposits. 
  In 
  many 
  places 
  the 
  rounded 
  gravel 
  and 
  sand 
  

   would 
  appear 
  to 
  graduate 
  on 
  the 
  same 
  horizon 
  into 
  clay 
  with 
  

   angular 
  stones 
  ; 
  but 
  in 
  most 
  places 
  it 
  evidently 
  comes 
  under 
  the 
  

   clay. 
  So 
  far 
  as 
  can 
  be 
  seen 
  in 
  sections, 
  the 
  clay 
  chiefly 
  occupies 
  

   hollows 
  (excepting 
  in 
  the 
  extensive 
  clay 
  flat 
  between 
  the 
  gravel 
  

   knolls 
  and 
  the 
  axial 
  ridge), 
  around 
  which 
  it 
  appears 
  to 
  thin 
  out 
  

   towards 
  the 
  summit 
  of 
  the 
  gravel 
  and 
  sand 
  knolls, 
  where 
  it 
  is 
  only 
  

   represented 
  by 
  a 
  sprinkling 
  of 
  loam, 
  with 
  local 
  angular 
  stones 
  

   (including 
  large 
  blocks), 
  which 
  are 
  every 
  year 
  becoming 
  scarcer 
  

   through 
  being 
  utilized 
  in 
  stone 
  walls. 
  In 
  trying 
  to 
  account 
  for 
  the 
  

   origin 
  of 
  the 
  knolls, 
  the 
  following 
  possible 
  explanations 
  may 
  be 
  

   briefly 
  taken 
  into 
  consideration 
  : 
  — 
  

  

  1. 
  Exposed 
  portions 
  of 
  Gravel 
  and 
  Sand 
  elsewhere 
  concealed 
  under 
  

   Clay 
  ? 
  — 
  Though 
  at 
  first 
  sight 
  this 
  theory 
  might 
  appear 
  satisfactory, 
  

   a 
  little 
  consideration 
  will 
  show 
  that 
  it 
  is 
  more 
  a 
  statement 
  of 
  a 
  fact 
  

   than 
  an 
  explanation, 
  because 
  it 
  does 
  not 
  account 
  for 
  the 
  steep- 
  

   sidedness 
  of 
  the 
  knolls, 
  or 
  for 
  their 
  frequent 
  occurrence 
  in 
  more 
  or 
  

   less 
  perched 
  positions. 
  

  

  2. 
  Heaps 
  of 
  erratic 
  stones 
  precipitated 
  from 
  floating 
  Ice? 
  — 
  As 
  

   most 
  of 
  the 
  stones 
  in 
  the 
  knolls 
  are 
  local, 
  or 
  iu 
  directly 
  local, 
  floating- 
  

   ice 
  carrying 
  stones 
  from 
  a 
  great 
  distance, 
  and 
  dropping 
  them 
  on 
  

   breaking 
  up 
  or 
  stranding, 
  would 
  not 
  account 
  for 
  the 
  existence 
  of 
  the 
  

   knolls, 
  though 
  it 
  is 
  conceivable 
  that 
  local 
  floating-ice, 
  on 
  becoming 
  

   charged 
  with 
  local 
  and 
  previously 
  deposited 
  erratic 
  stones, 
  might 
  

   have 
  formed 
  at 
  least 
  some 
  of 
  the 
  smaller 
  and 
  irregularly-shaped 
  

   knolls. 
  

  

  * 
  The 
  semicolon 
  in 
  line 
  4 
  from 
  the 
  top, 
  p. 
  365, 
  should 
  be 
  a 
  comma. 
  

  

  t 
  There 
  are, 
  however, 
  some 
  large 
  Arenig 
  boulders 
  which 
  probably 
  found 
  

   their 
  way 
  from 
  the 
  W. 
  over 
  the 
  Eglwyseg 
  escarpment. 
  It 
  would 
  appear 
  that 
  

   the 
  northern 
  drift 
  current 
  (at 
  a 
  height 
  of 
  more 
  than 
  1000 
  feet 
  above 
  the 
  

   present 
  sea-level) 
  must 
  have 
  terminated 
  before 
  reaching 
  as 
  far 
  south 
  as 
  Llangollen 
  

   Vale, 
  though 
  a 
  few 
  straggling 
  northern 
  erratics 
  may 
  yet 
  possibly 
  be 
  found 
  

   further 
  south. 
  

  

  