﻿Related 
  Sedimentary 
  Surface 
  Forms. 
  167 
  

  

  4. 
  Traveling. 
  — 
  degressive 
  sand- 
  waves, 
  as 
  stated 
  

   already, 
  owe 
  their 
  name 
  to 
  the 
  fact 
  that 
  they 
  travel 
  in 
  

   the 
  direction 
  opposite 
  to 
  that 
  of 
  ripples, 
  that 
  is 
  upstream, 
  

   some 
  of 
  the 
  sand 
  being 
  scoured 
  from 
  the 
  down-stream 
  

   face 
  and 
  deposited 
  on 
  the 
  up-stream 
  face 
  (Gilbert, 
  ibid., 
  

   p. 
  32). 
  

  

  This 
  progress 
  is, 
  however, 
  not 
  continuous, 
  since 
  all 
  

   regressive 
  sand-waves 
  in 
  nature 
  as 
  well 
  as 
  in 
  the 
  experi- 
  

   ment 
  are 
  subject 
  to 
  a 
  peculiar 
  rhythm, 
  which 
  Gilbert 
  

   describes 
  as 
  follows: 
  "Not 
  only 
  is 
  a 
  row 
  of 
  anti-dunes 
  

   a 
  rhythm 
  in 
  itself, 
  but 
  it 
  goes 
  through 
  a 
  rhythmic 
  fluctu- 
  

   ation 
  in 
  activity, 
  either 
  oscillating 
  above 
  a 
  mean 
  condition 
  

   or 
  else 
  developing 
  paroxysmally 
  on 
  a 
  plane 
  stream 
  bed 
  

   and 
  then 
  slowly 
  declining. 
  Paroxysmal 
  increase 
  starts 
  

   at 
  the 
  down-stream 
  end 
  of 
  a 
  row 
  and 
  travels 
  up-stream, 
  

   gaining 
  in 
  force 
  for 
  a 
  time, 
  and 
  the 
  climax 
  is 
  accompanied 
  

   by 
  a 
  combing 
  of 
  wave-crests. 
  Where 
  the 
  debris 
  is 
  very 
  

   coarse, 
  as 
  on 
  the 
  outwash 
  plains 
  of 
  glaciers, 
  a 
  din 
  of 
  

   crashing 
  bowlders 
  is 
  added 
  to 
  the 
  roar 
  of 
  the 
  water. 
  ' 
  ' 
  

  

  In 
  the 
  experimental 
  trough 
  some 
  of 
  these 
  waves 
  

   remained 
  "for 
  two 
  minutes 
  or 
  longer, 
  but 
  most 
  of 
  them 
  

   not 
  longer 
  than 
  one 
  minute. 
  " 
  " 
  Sometimes 
  two 
  or 
  more 
  

   will 
  disappear 
  at 
  once 
  and 
  leave 
  the 
  surface 
  without 
  

   waves 
  for 
  a 
  distance 
  of 
  ten 
  feet 
  or 
  more." 
  20 
  

  

  5. 
  Wave-length 
  and 
  amplitude. 
  — 
  In 
  streams 
  plowing 
  

   their 
  way 
  through 
  sandy 
  beaches 
  to 
  the 
  sea, 
  Cornish 
  

   observed 
  regressive 
  sand-waves, 
  measuring 
  but 
  23 
  cm. 
  

   from 
  crest 
  to 
  crest 
  (1899, 
  p. 
  626). 
  In 
  Gilbert's 
  experi- 
  

   ments 
  (1914, 
  p. 
  32), 
  their 
  wave-length 
  ranged 
  from 
  60 
  to 
  

   90 
  cm, 
  some 
  showing 
  amplitude 
  of 
  15 
  cm, 
  which 
  would 
  

   give 
  a 
  value 
  of 
  40 
  to 
  60 
  for 
  amplitude 
  over 
  wave-length. 
  

   With 
  different 
  grades 
  of 
  sand 
  (ib., 
  table 
  4) 
  they 
  were 
  

   observed 
  at 
  the 
  following 
  velocities 
  and 
  depths 
  : 
  

  

  Diameter 
  of 
  grains 
  Mean 
  velocity 
  Depth 
  

  

  (mm.) 
  (m./sec.) 
  (m.) 
  

  

  0-3 
  0-72-0-99 
  0-01-005 
  

  

  0-4 
  0-76-1-20 
  0-01-0-07 
  

  

  0-5 
  0-83-1-36 
  0-02-0-07 
  

  

  The 
  author 
  observed 
  the 
  water-waves 
  associated 
  with 
  

   these 
  sand-waves 
  during 
  a 
  cloudburst 
  at 
  the 
  mouth 
  of 
  a 
  

   small 
  stream 
  entering 
  the 
  Kentucky 
  River 
  a 
  few 
  miles 
  

   above 
  Worthville. 
  Their 
  wave-length 
  was 
  estimated 
  to 
  

  

  20 
  See 
  also 
  Pierce 
  's 
  observations. 
  

  

  