﻿Related 
  Sedimentary 
  Surface 
  Forms. 
  201 
  

  

  differing 
  considerably 
  in 
  the 
  distribution 
  of 
  energy 
  and 
  

   consequently 
  in 
  their 
  laws 
  of 
  existence, 
  nevertheless 
  

   appear 
  to 
  be 
  expressions 
  of 
  the 
  same 
  general 
  principle 
  

   which 
  we 
  may 
  now 
  give 
  in 
  this 
  form 
  : 
  A 
  sinuous 
  surface 
  

   of 
  contact 
  between 
  a 
  fluid 
  and 
  a 
  sediment 
  offers 
  a 
  mini- 
  

   mum 
  of 
  friction. 
  The 
  details 
  of 
  the 
  form 
  of 
  this 
  contact 
  

   surface 
  in 
  each 
  case 
  are 
  defined 
  by 
  the 
  controlling 
  condi- 
  

   tions. 
  It 
  even 
  is 
  not 
  impossible 
  that 
  more 
  than 
  one 
  curve 
  

   will 
  satisfy 
  certain 
  conditions. 
  , 
  

  

  We 
  may 
  now, 
  for 
  a 
  moment, 
  turn 
  back 
  to 
  the 
  problem 
  

   of 
  the 
  origin 
  of 
  dunes. 
  Since 
  the 
  definition 
  of 
  the 
  term 
  

   "dune" 
  does 
  not 
  include 
  a 
  rhythmic 
  repetition 
  of 
  the 
  

   pattern, 
  there 
  are, 
  in 
  reality, 
  two 
  quite 
  different 
  processes 
  

   involved, 
  viz. 
  the 
  origin 
  of 
  isolated 
  dune 
  hills, 
  and 
  the 
  

   origin 
  of 
  the 
  rhythmic 
  repetition 
  of 
  the 
  dune-pattern 
  

   characteristic 
  of 
  the 
  larger 
  dune 
  regions. 
  

  

  The 
  first 
  problem 
  does 
  not 
  directly 
  concern 
  us 
  here. 
  

   Any 
  obstacle 
  in 
  the 
  way 
  of 
  sand- 
  transporting 
  wind, 
  either 
  

   a 
  foreign 
  object, 
  65 
  or 
  a 
  primary 
  irregular 
  accumulation 
  

   of 
  the 
  sand 
  itself, 
  66 
  is 
  bound 
  to 
  collect 
  sand 
  around 
  itself 
  

   and 
  hold 
  it 
  under 
  the 
  influence 
  of 
  the 
  suction 
  eddy 
  on 
  the 
  

   lee-side, 
  which 
  under 
  ordinary 
  wind 
  conditions, 
  does 
  

   not 
  allow 
  the 
  &and-grains 
  to 
  escape 
  from 
  their 
  hold! 
  67 
  

   This 
  process 
  obviously 
  is 
  independent 
  of 
  the 
  size 
  of 
  the 
  

   obstacle. 
  Whether, 
  however, 
  an 
  initial 
  dune 
  will 
  persist 
  

   or 
  be 
  "blown" 
  away, 
  depends 
  on 
  its 
  rate 
  of 
  growth 
  and 
  

   the 
  velocity 
  and 
  direction 
  of 
  the 
  strongest 
  winds. 
  

  

  That 
  the 
  rate 
  of 
  growth 
  of 
  wind-formed 
  current-rip- 
  

   ples 
  is 
  not 
  sufficient 
  to 
  allow 
  them 
  to 
  gradually 
  grow 
  up 
  

   into 
  "dunes" 
  68 
  is 
  shown 
  conclusively 
  by 
  experimental 
  

   evidence 
  (cf. 
  p. 
  196). 
  The 
  regular 
  systems 
  of 
  trans- 
  

   verse 
  dunes 
  can, 
  therefore, 
  not 
  be 
  explained 
  as 
  full- 
  

   grown 
  ripples. 
  Their 
  formation 
  becomes, 
  however, 
  

   intelligible 
  in 
  the 
  light 
  of 
  King's 
  observations 
  and 
  experi- 
  

   ments, 
  which 
  prove 
  that 
  corresponding 
  to 
  every 
  initial 
  

   obstacle, 
  for 
  instance 
  a 
  dune, 
  of 
  given 
  size 
  and 
  wind 
  of 
  

   sufficient 
  velocity 
  there 
  exists 
  a 
  sinuous 
  surface 
  which, 
  

  

  65 
  Vegetation, 
  stones, 
  minor 
  irregularities 
  of 
  underlying 
  topography, 
  etc. 
  

  

  6P 
  Due 
  to 
  irregular 
  deposition 
  of 
  sand 
  carried 
  in 
  suspension 
  in 
  sand- 
  

   storms; 
  to 
  unequal 
  distribution 
  of 
  moisture 
  in 
  a 
  sandsurface 
  being 
  tempo- 
  

   rarily 
  eroded, 
  etc., 
  etc. 
  

  

  C7 
  A 
  land 
  surface 
  may 
  be 
  entirely 
  stripped 
  of 
  loose 
  sand 
  by 
  frequent 
  

   strong 
  winds, 
  only 
  such 
  sand 
  being 
  retained 
  which 
  happens 
  to 
  be 
  caught 
  

   on 
  a 
  dune. 
  This 
  "curious 
  power' 
  ' 
  of 
  dunes 
  "for 
  collecting 
  all 
  the 
  sand 
  

   in 
  the 
  neighborhood" 
  was 
  recently 
  discussed 
  by 
  King 
  (1. 
  c, 
  p. 
  207). 
  

  

  68 
  As 
  suggested 
  by 
  Cornish, 
  for 
  example 
  (Geogr. 
  Journ., 
  1907). 
  

  

  