﻿Related 
  Sedimentary 
  Surface 
  Forms. 
  175 
  

  

  This 
  arrangement 
  of 
  the 
  sand-waves 
  is 
  in 
  itself 
  the 
  best 
  

   proof 
  of 
  their 
  current 
  origin. 
  26 
  That 
  wind-drift 
  may 
  be 
  

   a 
  strong 
  factor 
  in 
  the 
  formation 
  of 
  sand-waves 
  of 
  this 
  

   size 
  seems 
  to 
  be 
  proved 
  by 
  the 
  following 
  observation 
  : 
  

  

  Cornish 
  visited 
  Pegwell 
  Bay 
  on 
  an 
  afternoon 
  previous 
  

   to 
  which 
  the 
  wind 
  had 
  been 
  blowing 
  into 
  the 
  Bay 
  for 
  at 
  

   least 
  twenty-four 
  hours 
  with 
  a 
  force 
  4 
  of 
  the 
  scale 
  0-12 
  r 
  

   increasing 
  during 
  the 
  night 
  to 
  force 
  6. 
  ' 
  ' 
  There 
  had 
  been 
  

   a 
  strong 
  current 
  setting 
  into 
  the 
  Bay 
  in 
  the 
  morning, 
  and 
  

   the 
  flats 
  of 
  sticky 
  sand 
  exposed 
  at 
  low 
  water 
  were 
  covered 
  

   with 
  tidal 
  sand 
  ridges 
  of 
  about 
  8' 
  wave 
  length 
  and 
  3" 
  

   amplitude 
  ... 
  In 
  calm 
  weather, 
  I 
  am 
  informed 
  these 
  

   sands 
  are 
  smooth, 
  except 
  for 
  the 
  ordinary 
  small 
  ripple 
  

   mark. 
  This 
  observation 
  shows 
  that 
  "tidal" 
  sand- 
  

   ridges 
  can 
  be 
  formed 
  by 
  a 
  drift 
  of 
  water 
  caused 
  by 
  the 
  

   wind 
  setting 
  into 
  a 
  bay" 
  (p. 
  190). 
  

  

  2. 
  Profile 
  and 
  structure. 
  — 
  "In 
  well-developed 
  ridges 
  

   the 
  summit 
  is 
  not 
  coincident 
  with 
  the 
  top 
  of 
  the 
  cliff, 
  but 
  

   some 
  distance 
  on 
  the 
  weather 
  side, 
  which 
  is 
  slightly 
  con- 
  

   cave 
  in 
  its 
  lower 
  and 
  convex 
  in 
  its 
  upper 
  portion" 
  (p. 
  

   171). 
  This 
  gives 
  the 
  top 
  of 
  the 
  ridges 
  a 
  rounded 
  appear- 
  

   ance. 
  

  

  "Symmetrical 
  ridges 
  with 
  both 
  faces 
  sloping 
  at 
  about 
  

   12° 
  are 
  very 
  rare 
  above 
  low 
  water 
  mark; 
  generally 
  the 
  

   slower 
  reverse 
  current 
  appears 
  to 
  have 
  no 
  effect 
  upon 
  

   the 
  form 
  of 
  the 
  ridges." 
  

  

  These 
  large 
  ripples, 
  of 
  course, 
  face 
  with 
  the 
  current 
  

   that 
  produces 
  them, 
  with 
  the 
  flood 
  current 
  where 
  the 
  ebb- 
  

   current 
  is 
  too 
  weak 
  to 
  change 
  their 
  form, 
  otherwise 
  with 
  

   the 
  latter. 
  Occasionally 
  the 
  same 
  estuary 
  at 
  low-water 
  

   will 
  show 
  on 
  different 
  parts 
  of 
  its 
  exposed 
  flats 
  ripples 
  

   facing 
  in 
  opposite 
  directions 
  (p. 
  171). 
  Cornish 
  also 
  

   observed 
  large 
  ripples 
  facing 
  up-stream 
  covered 
  with 
  

   ordinary 
  current-ripples 
  facing 
  with 
  the 
  ebb 
  (p. 
  174), 
  

   which 
  was 
  not 
  able 
  to 
  reverse 
  them. 
  In 
  general, 
  current- 
  

   ripples 
  may 
  cross 
  these 
  sand-waves 
  at 
  any 
  angle. 
  

  

  Cornish 
  gives 
  little 
  information 
  concerning 
  the 
  mechan- 
  

   ical 
  composition 
  of 
  the 
  material 
  of 
  these 
  sand-waves. 
  

   The 
  terms 
  "sand," 
  "sticky 
  sand" 
  and 
  mud-flats" 
  

   occur 
  repeatedly 
  ; 
  nothing, 
  however, 
  enables 
  us 
  to 
  decide 
  

   whether 
  these 
  terms 
  apply 
  to 
  the 
  sediment 
  as 
  a 
  whole 
  or 
  

  

  26 
  These 
  sand-waves 
  of 
  the 
  Goodwin 
  Sands 
  with 
  an 
  amplitude 
  of 
  90 
  cm. 
  

   have 
  repeatedly 
  been 
  quoted 
  as 
  an 
  example 
  of 
  the 
  supposed 
  maximum 
  

   dimensions 
  of 
  wave-formed 
  ripples. 
  

  

  