﻿Related 
  Sedimentary 
  Surface 
  Forms. 
  165 
  

  

  II. 
  Sand-waves 
  and 
  Related 
  Surface 
  Forms. 
  

   1. 
  Second 
  critical 
  point 
  of 
  current 
  velocity. 
  

  

  At 
  a 
  certain 
  velocity, 
  which 
  differs 
  for 
  each 
  grade 
  of 
  

   debris, 
  the 
  mode 
  of 
  transportation 
  is 
  changed 
  and 
  the 
  

   current-ripples 
  disappear. 
  "As 
  the 
  velocity 
  of 
  the 
  cur- 
  

   rent 
  increases 
  some 
  of 
  the 
  grains 
  leap 
  as 
  well 
  as 
  roll, 
  

   and 
  some, 
  instead 
  of 
  dropping 
  over 
  the 
  crest 
  of 
  a 
  dune 
  

   and 
  resting, 
  leap 
  to 
  the 
  next 
  dune. 
  The 
  dune 
  grows 
  less 
  

   distinct 
  in 
  form 
  and 
  finally 
  at 
  a 
  critical 
  velocity 
  it 
  dis- 
  

   appears, 
  dune 
  motion 
  ceases, 
  and 
  the 
  sand 
  surface 
  

   becomes 
  comparatively 
  even." 
  18 
  Correspondingly, 
  Ber- 
  

   thololy 
  observed 
  that 
  in 
  creeks, 
  with 
  increasing 
  volume, 
  

   that 
  is 
  velocity, 
  of 
  the 
  water 
  all 
  current-ripples 
  dis- 
  

   appeared 
  in 
  certain 
  places 
  (1900, 
  p. 
  92). 
  

  

  This 
  is 
  due 
  to 
  the 
  fact 
  that 
  the 
  mechanical 
  effect 
  of 
  the 
  

   current 
  is 
  extended 
  below 
  the 
  surface, 
  destroying 
  the 
  

   immobility 
  of 
  the 
  sand 
  bed, 
  which 
  is 
  the 
  prerequisite 
  of 
  

   the 
  existence 
  of 
  current-ripples. 
  Instead 
  of 
  the 
  topmost 
  

   grains 
  rolling 
  and 
  skipping 
  over 
  a 
  bed 
  of 
  sand, 
  we 
  find 
  

   a 
  whole 
  layer 
  of 
  mixed 
  sand 
  and 
  water 
  in 
  motion, 
  grad- 
  

   ing 
  insensibly 
  into 
  the 
  motionless 
  substratum. 
  Above 
  

   this 
  layer 
  of 
  moving 
  sand 
  and 
  water 
  we 
  find 
  water 
  with 
  

   little 
  sand 
  in 
  suspension, 
  the 
  transition 
  not 
  being 
  gradual 
  

   but 
  abrupt 
  (cf. 
  Gilbert, 
  1914, 
  ^g. 
  5, 
  p. 
  27). 
  

  

  A. 
  Regressive 
  Sand-waves. 
  

   1. 
  The 
  third 
  critical 
  point. 
  

  

  After 
  the 
  second 
  critical 
  point 
  there 
  followed 
  in 
  

   Gilbert's 
  experiments 
  a 
  "smooth 
  phase," 
  in 
  which 
  the 
  

   sand 
  surface 
  was 
  comparatively 
  even, 
  the 
  motion 
  of 
  the 
  

   sand 
  extending 
  quite 
  a 
  distance 
  below 
  the 
  surface. 
  With 
  

   increasing 
  velocity 
  a 
  third 
  critical 
  point 
  was 
  reached 
  at 
  

   which 
  the 
  sand 
  surface 
  again 
  assumed 
  a 
  waved 
  appear- 
  

   ance. 
  These 
  new 
  sand-waves, 
  in 
  contrast 
  to 
  current- 
  

   ripples, 
  traveled 
  up-stream. 
  To 
  emphasize 
  this 
  contrast 
  

   Gilbert 
  called 
  them 
  "anti- 
  dunes." 
  For 
  reasons 
  to 
  be 
  

   given 
  later, 
  this 
  concise 
  term 
  is 
  not 
  used 
  in 
  this 
  paper, 
  

   but 
  for 
  it 
  is 
  substituted 
  that 
  of 
  "regressive 
  sand-waves." 
  

   Similar 
  sand-waves 
  are 
  common 
  in 
  shallow, 
  silt-laden 
  

   streams 
  and 
  have 
  repeatedly 
  been 
  described 
  in 
  literature. 
  

   They 
  will 
  be 
  discussed 
  with 
  those 
  observed 
  in 
  the 
  experi- 
  

   ment. 
  

  

  18 
  Murphy 
  in 
  Gilbert, 
  1914, 
  p. 
  32. 
  

  

  Am. 
  Jour. 
  Sci.— 
  Fourth 
  Series, 
  Vol. 
  XL 
  VII, 
  No. 
  279.— 
  March, 
  1919. 
  

   12 
  

  

  