﻿342 
  J. 
  Bar 
  veil 
  — 
  The 
  Piedmont 
  Terraces 
  

  

  ceases 
  where 
  wind 
  gaps 
  occur 
  and 
  can 
  not 
  be 
  renewed 
  at 
  

   these 
  places 
  in 
  the 
  next 
  cycle 
  because 
  of 
  the 
  diversion 
  of 
  

   the 
  channeling 
  stream. 
  A 
  competent 
  stream 
  will 
  sink 
  its 
  

   gorge 
  through 
  a 
  resistant 
  formation 
  in 
  a 
  small 
  fraction 
  

   of 
  the 
  time 
  that 
  surface 
  wash 
  requires 
  to 
  lower 
  the 
  same 
  

   formation 
  an 
  equal 
  amount. 
  The 
  gorge 
  may 
  thus 
  be 
  the 
  

   only 
  mark 
  of 
  a 
  baselevel 
  on 
  a 
  resistant 
  ridge 
  and 
  it 
  will 
  

   endure 
  while 
  the 
  whole 
  surface 
  of 
  adjacent 
  soft 
  forma- 
  

   tions 
  is 
  repeatedly 
  baseleveled 
  in 
  succeeding 
  cycles. 
  In 
  

   this 
  respect, 
  therefore, 
  the 
  cutting 
  of 
  wind 
  gaps 
  is 
  a 
  flu- 
  

   vial 
  process 
  which 
  corresponds 
  to 
  wave 
  planation 
  against 
  

   residuals 
  of 
  resistant 
  rock 
  during 
  a 
  submergent 
  phase 
  at 
  

   the 
  close 
  of 
  a 
  cycle 
  of 
  marine 
  peneplanation. 
  

  

  Significance 
  of 
  Present 
  Valley 
  Forms. 
  

  

  Let 
  the 
  geologist 
  travel 
  up 
  a 
  river 
  which 
  is 
  entrenched 
  

   to 
  a 
  depth 
  of 
  fixe 
  hundred 
  to 
  one 
  thousand 
  feet 
  in 
  resist- 
  

   ant 
  rock, 
  as 
  are 
  the 
  Housatonic, 
  the 
  Hudson 
  through 
  the 
  

   Highlands, 
  or 
  the 
  upper 
  courses 
  of 
  the 
  Delaware 
  and 
  

   Susquehanna. 
  From 
  the 
  railroad 
  or 
  highway 
  he 
  will 
  be 
  

   most 
  impressed 
  by 
  the 
  precipitous 
  slopes 
  which 
  rise 
  some 
  

   hundreds 
  of 
  feet 
  above 
  him. 
  Let 
  him 
  climb 
  some 
  hill 
  

   from 
  which 
  he 
  may 
  gain 
  a 
  commanding 
  view 
  along 
  the 
  

   main 
  valley 
  or 
  up 
  those 
  of 
  tributary 
  streams. 
  He 
  will 
  

   commonly 
  see 
  the 
  outlines 
  of 
  an 
  upper 
  and 
  wider 
  valley 
  

   with 
  gentle 
  slopes 
  below 
  which 
  the 
  present 
  stream 
  is 
  en- 
  

   trenched. 
  From 
  especially 
  favorable 
  situations 
  he 
  may 
  

   obtain 
  an 
  impression 
  of 
  a 
  still 
  higher 
  and 
  flatter 
  valley 
  

   outline. 
  He 
  will 
  put 
  together 
  the 
  scattered 
  bits 
  of 
  evi- 
  

   dence 
  and 
  reach 
  a 
  convincing 
  conclusion 
  because 
  he 
  can 
  

   see 
  that 
  the 
  tops 
  of 
  various 
  spurs 
  and 
  outlying 
  hills 
  fall 
  

   into 
  the 
  same 
  level 
  plane. 
  

  

  The 
  significance 
  of 
  such 
  valley 
  slopes 
  is 
  now 
  well 
  rec- 
  

   ognized 
  by 
  physiographers 
  ; 
  the 
  upper 
  and 
  flatter 
  slopes 
  

   indicate 
  a 
  stillstand 
  of 
  the 
  land 
  long 
  enough 
  for 
  a 
  valley 
  

   in 
  hard 
  rock 
  to 
  reach 
  old 
  age, 
  followed 
  by 
  uplift 
  so 
  recent 
  

   that 
  the 
  river 
  has 
  had 
  time 
  only 
  to 
  carve 
  a 
  young 
  valley 
  

   in 
  the 
  old 
  valley 
  floor. 
  There 
  are 
  certain 
  aspects 
  of 
  

   these 
  valley 
  forms, 
  however, 
  which 
  are 
  not 
  so 
  thoroughly 
  

   appreciated 
  by 
  geologists 
  in 
  general 
  and 
  these 
  need 
  dis- 
  

   cussion 
  in 
  this 
  section 
  of 
  the 
  paper 
  dealing 
  with 
  the 
  

   development 
  of 
  principles 
  and 
  criteria. 
  

  

  These 
  valley 
  facets 
  are 
  seldom 
  well 
  shown 
  on 
  topo- 
  

   graphic 
  maps; 
  their 
  representation 
  requires 
  a 
  point 
  of 
  

  

  