﻿Related 
  Sedimentary 
  Surface 
  Forms. 
  151 
  

  

  debris 
  that 
  were 
  found 
  undisturbed 
  at 
  their 
  bottom. 
  

   Login 
  and 
  Gilbert 
  worked 
  with 
  artificial 
  channels. 
  The 
  

   abbreviated 
  form 
  in 
  which 
  Gilbert's 
  figures 
  are 
  quoted 
  

   does 
  not 
  do 
  justice 
  to 
  the 
  comprehensiveness 
  of 
  his 
  data. 
  

   Umpfenbach, 
  in 
  contrast 
  to 
  Login 
  and 
  Gilbert, 
  deter- 
  

   mined 
  the 
  surface 
  velocities 
  at 
  which 
  the 
  debris 
  remained 
  

   unmoved 
  in 
  natural 
  water 
  courses. 
  He 
  probably 
  dealt 
  

   with 
  depths 
  measured 
  in 
  decimeters 
  rather 
  than 
  in 
  cen- 
  

   timeters, 
  which 
  will 
  account 
  for 
  the 
  greater 
  velocities 
  

   found. 
  None 
  of 
  them 
  give 
  bed 
  velocity 
  ; 
  that 
  is, 
  the 
  mean 
  

   of 
  such 
  velocities 
  as 
  occur 
  sufficiently 
  near 
  the 
  bed 
  to 
  be 
  

   directly 
  engaged 
  in 
  the 
  process 
  of 
  traction. 
  

  

  Table 
  I, 
  Showing 
  Approximate 
  Current 
  Velocities 
  Necessary 
  to 
  move 
  

  

  Debris 
  of 
  Different 
  Size. 
  

  

  Mean 
  diameter 
  Depth 
  Velocity 
  

  

  Description 
  mm. 
  m. 
  m./sec. 
  

   Brick 
  clay, 
  allowed 
  to 
  settle 
  from 
  

  

  suspension 
  (L) 
  (shallow) 
  0-08 
  (s) 
  

  

  Fine 
  loam 
  and 
  mud 
  (U) 
  (shallow) 
  0-32 
  (s) 
  

  

  (Fine 
  sand) 
  (G) 
  0-4 
  0-13 
  0-26 
  (to) 
  

  

  (Sand) 
  (G) 
  05 
  0-12 
  0-28 
  (to) 
  

  

  Freshwater 
  sand 
  (L) 
  (shallow) 
  0-20 
  (s) 
  

  

  (Sand) 
  (G) 
  0-7 
  0-2 
  0-34 
  (to) 
  

  

  Sea 
  sand 
  (L) 
  . 
  .- 
  .- 
  (shallow) 
  0-34 
  (s) 
  

  

  Coarse 
  sand 
  (U) 
  (shallow) 
  0-49 
  (s) 
  

  

  (Coarse 
  sand) 
  (G) 
  1-7 
  0-006 
  0-34 
  (to) 
  

  

  (Fine 
  gravel) 
  (G) 
  3-2 
  0-028 
  0-46 
  (to) 
  

  

  Bounded 
  pebbles, 
  size 
  of 
  peas 
  (L) 
  . 
  . 
  (shallow) 
  0-61 
  (s) 
  

  

  Very 
  small 
  pebbles 
  (U) 
  (shallow) 
  0-65 
  (s) 
  

  

  (Fine 
  gravel) 
  (G) 
  4-9 
  0-033 
  0-65 
  (to) 
  

  

  (Fine 
  gravel) 
  (G) 
  7-0 
  0-066 
  0-86 
  (to) 
  

  

  Gravel 
  (U) 
  27-0 
  (shallow) 
  0-97 
  (s) 
  

  

  Gravel 
  (U) 
  54-0 
  (shallow) 
  1-62 
  (s) 
  

  

  Bowlders 
  (U) 
  171-0 
  (shallow) 
  2-27 
  (s) 
  

  

  Bowlders 
  (U) 
  323-0 
  (shallow) 
  3-25 
  (s) 
  

  

  Bowlders 
  (U) 
  409-0 
  (shallow) 
  4-87 
  (s) 
  

  

  Bowlders 
  (U) 
  700-800-0 
  (shallow) 
  11-69 
  (s) 
  

  

  Note. 
  

  

  (U) 
  — 
  Umpfenbach, 
  quoted 
  from 
  Penck, 
  Morphologie 
  der 
  Erdobernaeche, 
  

   vol. 
  1, 
  p. 
  283, 
  1894. 
  

  

  (L) 
  — 
  Login, 
  Proc. 
  Eoy. 
  Soc, 
  Edinburgh, 
  vol. 
  3, 
  p. 
  475, 
  1857. 
  

  

  (G) 
  — 
  Gilbert 
  (1914) 
  ; 
  the 
  first 
  three 
  figures 
  taken 
  from 
  table 
  9, 
  p. 
  69; 
  

   the 
  others 
  from 
  the 
  averages 
  given 
  on 
  p. 
  71. 
  

  

  Also: 
  (5) 
  — 
  surface 
  velocity, 
  (to) 
  = 
  mean 
  velocity. 
  Login's 
  and 
  Gil- 
  

   bert's 
  figures 
  have 
  been 
  transformed 
  from 
  inches 
  to 
  nearest 
  mm., 
  and 
  from 
  

   feet 
  to 
  nearest 
  cm. 
  

  

  2. 
  The 
  first 
  appearance 
  of 
  cur 
  rent 
  -ripples. 
  

  

  Soon 
  after 
  a 
  large 
  number 
  of 
  grains 
  have 
  begun 
  to 
  

   move, 
  often 
  immediately 
  after 
  the 
  first 
  movement 
  set 
  in, 
  

   a 
  rippling 
  appears 
  on 
  the 
  surface 
  of 
  the 
  sand. 
  

  

  