﻿Related 
  Sedimentary 
  Surface 
  Forms. 
  155 
  

  

  most 
  typical 
  case, 
  consist 
  of 
  laminae 
  of 
  deposition 
  parallel 
  

   to 
  the 
  lee-slope, 
  cut 
  at 
  an 
  oblique 
  angle 
  by 
  the 
  up-stream 
  

   slope. 
  Where 
  the 
  transported 
  material 
  consists 
  of 
  a 
  

   very 
  inhomogeneous 
  mixture, 
  this 
  structure 
  is 
  easily 
  dis- 
  

   cerned. 
  Berthololy, 
  for 
  example 
  (1900, 
  p. 
  99), 
  observed 
  

   the 
  following 
  layers 
  in 
  a 
  single 
  current-ripple 
  on 
  the 
  dry 
  

   bed 
  of 
  a 
  creek: 
  sand, 
  humus, 
  mud, 
  mud 
  and 
  sand, 
  sand. 
  

   This 
  structure 
  can 
  be 
  preserved 
  only 
  if 
  the 
  direction 
  of 
  

   the 
  current 
  remains 
  constant 
  for 
  sufficient 
  length 
  of 
  time. 
  

   Otherwise 
  the 
  slopes 
  of 
  the 
  new 
  ripple 
  surface 
  will 
  inter- 
  

   sect 
  with 
  the 
  lines 
  of 
  sedimentation 
  of 
  the 
  older 
  ones, 
  

   producing 
  a 
  complex 
  pattern. 
  This 
  I 
  have 
  very 
  fre- 
  

   quently 
  observed, 
  e. 
  g., 
  in 
  the 
  shaly 
  sandstones 
  of 
  the 
  

   Pottsville 
  group 
  in 
  eastern 
  Kentucky 
  and 
  also 
  in 
  some 
  

   rippled 
  limestones 
  of 
  the 
  Richmond 
  group 
  in 
  central 
  Ken- 
  

   tucky. 
  

  

  Berthololy 
  observed 
  also 
  that 
  in 
  active 
  current-ripples 
  

   the 
  coarser 
  grains 
  accumulated 
  on 
  the 
  lee-side. 
  10 
  In 
  fact, 
  

   the 
  largest 
  grains 
  on 
  the 
  lee-side 
  exceeded 
  those 
  on 
  the 
  

   stoss-side 
  three 
  to 
  four 
  times 
  (1900, 
  p. 
  96), 
  probably 
  

   because 
  the 
  smaller 
  grains 
  are 
  caught 
  oftener 
  in 
  their 
  

   march 
  across 
  the 
  upstream 
  side, 
  therefore 
  traveling 
  

   slower 
  than 
  the 
  coarser 
  grains, 
  which 
  are 
  given 
  a 
  longer 
  

   rest 
  on 
  the 
  lee-side 
  (1900, 
  p. 
  98). 
  Similar 
  discrepancies 
  

   in 
  size 
  of 
  the 
  grains 
  on 
  the 
  two 
  sides 
  of 
  current-ripples 
  

   were 
  frequently 
  observed 
  by 
  the 
  author. 
  

  

  Two 
  measurements 
  of 
  the 
  speed 
  of 
  traveling 
  are 
  given 
  

   in 
  tables 
  2 
  and 
  3. 
  This 
  speed 
  is 
  greatly 
  increased, 
  if 
  

   during 
  the 
  marching 
  of 
  the 
  current-ripples 
  material 
  is 
  

   added 
  from 
  above. 
  In 
  this 
  case, 
  sediment 
  is 
  deposited 
  

   on 
  both 
  stoss- 
  and 
  lee-side, 
  although 
  much 
  more 
  on 
  the 
  

   latter 
  than 
  on 
  the 
  former, 
  resulting 
  in 
  a 
  peculiarly 
  small 
  

   scale 
  pattern 
  of 
  cross-bedding, 
  which 
  Sorby 
  has 
  called 
  

   ripple-drift. 
  11 
  

  

  Since 
  the 
  current 
  in 
  contact 
  with 
  the 
  sediment 
  must 
  be 
  

   so 
  weak 
  as 
  not 
  to 
  carry 
  any 
  sediment 
  in 
  suspension 
  in 
  

   order 
  to 
  give 
  rise 
  to 
  current-ripples, 
  no 
  sediment 
  can 
  be 
  

   added 
  through 
  it 
  directly 
  to 
  the 
  rippled 
  sedimentary 
  sur- 
  

   face. 
  The 
  preservation 
  of 
  current-ripples 
  in 
  general, 
  

  

  10 
  Similar 
  conditions 
  were 
  observed 
  on 
  eolian 
  current-ripples. 
  Forchham- 
  

   mer 
  (1841, 
  p. 
  7) 
  saw 
  on 
  dunes 
  of 
  Northern 
  Jutland 
  the 
  crests 
  of 
  eolian 
  

   current-ripples 
  covered 
  with 
  white 
  quartz 
  sand, 
  while 
  the 
  hollows 
  consisted 
  

   of 
  black 
  magnetite 
  sand. 
  

  

  11 
  1859, 
  p. 
  143. 
  I 
  suspect 
  that 
  much 
  of 
  what 
  Gilbert 
  described 
  and 
  

   figured 
  as 
  "Compound 
  cross-bedding" 
  (Bull. 
  Geol. 
  Soc. 
  Am., 
  vol. 
  10, 
  

   p. 
  139, 
  pp. 
  3-5, 
  1899) 
  is 
  ripple-drift. 
  

  

  