﻿348 
  J. 
  Barrell 
  — 
  The 
  Piedmont 
  Terraces 
  

  

  order 
  that 
  the 
  successive 
  profiles 
  should 
  be 
  cut, 
  the 
  

   younger 
  within 
  the 
  older, 
  the 
  oldest 
  cycle, 
  C, 
  must 
  have 
  

   been 
  the 
  longest, 
  E 
  the 
  next 
  longest, 
  and 
  F 
  the 
  most 
  brief. 
  

   This 
  kind 
  of 
  a 
  record 
  implies 
  an 
  increasing 
  rapidity 
  of 
  

   recurrence 
  in 
  diastrophism. 
  As 
  this 
  is 
  a 
  conclusion 
  with 
  

   far-reaching 
  implication 
  let 
  us 
  examine 
  some 
  other 
  points 
  

   of 
  view. 
  

  

  If 
  the 
  present 
  baselevel 
  should 
  endure 
  for 
  a 
  sufficient 
  

   time 
  the 
  streams 
  would 
  sink 
  to 
  a 
  flatter 
  grade, 
  the 
  valley 
  

   slopes 
  would 
  become 
  more 
  gentle, 
  and 
  the 
  profile 
  F 
  

   change 
  to 
  F', 
  destroying 
  the 
  previous 
  record. 
  But 
  as 
  we 
  

   can 
  not 
  look 
  into 
  the 
  future, 
  the 
  nature 
  of 
  the 
  series 
  must 
  

   be 
  decided 
  on 
  the 
  basis 
  of 
  the 
  record 
  of 
  the 
  past. 
  That 
  

   record 
  is 
  shown 
  by 
  the 
  relations 
  of 
  A, 
  C, 
  and 
  E, 
  and 
  these 
  

   indicate 
  an 
  increasing 
  rapidity 
  in 
  the 
  recurrence 
  of 
  

   uplifts. 
  These 
  uplifts, 
  however, 
  need 
  not 
  have 
  been 
  

   simple 
  and 
  free 
  from 
  short 
  period 
  minor 
  oscillations. 
  

   As 
  shown 
  by 
  the 
  broken 
  lines 
  in 
  fig. 
  13, 
  a 
  short 
  pause, 
  B, 
  

   may 
  have 
  occurred 
  and 
  yet 
  had 
  the 
  evidence 
  of 
  its 
  exist- 
  

   ence 
  totally 
  destroyed 
  by 
  a 
  longer 
  pause 
  C, 
  and 
  another,. 
  

   D, 
  by 
  a 
  longer 
  pause 
  E. 
  Even 
  the 
  development 
  of 
  pene- 
  

   plains 
  does 
  not 
  require 
  an 
  age-long 
  freedom 
  from 
  move- 
  

   ments 
  in 
  baselevel. 
  Minor 
  oscillations 
  may 
  continue, 
  but 
  

   they 
  need 
  not 
  lead 
  to 
  a 
  progressive 
  rise 
  of 
  the 
  land. 
  In 
  

   the 
  lower 
  portions 
  of 
  the 
  river 
  systems 
  alternate 
  fill 
  and 
  

   scour 
  of 
  the 
  valley 
  would 
  take 
  place, 
  but 
  in 
  the 
  upper 
  

   parts 
  or 
  on 
  the 
  interfluvial 
  slopes 
  erosion 
  would 
  still 
  con- 
  

   tinue 
  and 
  increase 
  the 
  perfection 
  of 
  the 
  peneplain. 
  

  

  The 
  significance 
  of 
  such 
  a 
  quickening 
  series 
  of 
  uplifts 
  

   in 
  relation 
  to 
  the 
  increase 
  in 
  the 
  rate 
  of 
  fluvial 
  erosion 
  in 
  

   recent 
  geologic 
  time 
  and 
  the 
  vast 
  stretching 
  out 
  which 
  it 
  

   implies 
  for 
  the 
  earlier 
  stages 
  of 
  erosion 
  has 
  been 
  dis- 
  

   cussed 
  by 
  the 
  writer 
  elsewhere, 
  23 
  the 
  argument 
  resting 
  

   especially 
  upon 
  the 
  Pleistocene 
  series 
  of 
  uplifts 
  as 
  

   recorded 
  on 
  the 
  soft 
  deposits 
  of 
  the 
  Coastal 
  Plain. 
  The 
  

   same 
  argument 
  is 
  seen 
  to 
  apply 
  to 
  the 
  series 
  of 
  higher 
  

   baselevels 
  recorded 
  upon 
  more 
  resistant 
  formations 
  and 
  

   reaching 
  back 
  of 
  the 
  Pleistocene 
  into 
  the 
  Tertiary. 
  

  

  The 
  evidence 
  of 
  the 
  valley 
  forms 
  shows 
  that 
  we 
  live 
  in 
  

   an 
  age 
  of 
  quickening 
  earth 
  unrest 
  and 
  that 
  geologic 
  time 
  

   is 
  long, 
  to 
  a 
  degree 
  which 
  even 
  now 
  is 
  but 
  little 
  appre- 
  

   ciated. 
  

  

  23 
  Loc. 
  cit., 
  pp. 
  761-767. 
  

  

  