﻿J. 
  Barrell 
  — 
  Piedmont 
  Terraces. 
  407 
  

  

  Art. 
  XXXII. 
  — 
  The 
  Piedmont 
  Terraces 
  of 
  the 
  Northern 
  

   Appalachians; 
  by 
  Joseph 
  Barrell. 
  With 
  Plates 
  V 
  

   and 
  VI. 
  

  

  Edited 
  by 
  H. 
  H. 
  Eobinson. 
  

   [Continued 
  from 
  p. 
  362] 
  

  

  Criteria 
  for 
  recognition 
  of 
  fluvial 
  and 
  marine 
  baselevels. 
  

  

  Outcrops 
  of 
  Unconformities. 
  

  

  It 
  is 
  evident 
  that 
  the 
  problem 
  of 
  tracing 
  a 
  system 
  of 
  

   baselevels 
  is 
  distinctly 
  complex. 
  Where 
  successive 
  base- 
  

   levels 
  are 
  present, 
  criteria 
  must 
  be 
  used, 
  therefore, 
  which 
  

   will 
  give 
  the 
  elevation 
  of 
  each 
  one 
  with 
  a 
  maximum 
  de- 
  

   gree 
  of 
  certainty. 
  The 
  baselevels 
  must 
  be 
  traced 
  from 
  

   place 
  to 
  place 
  and 
  correlated 
  by 
  elevation, 
  gradient, 
  and 
  

   similarity 
  of 
  record 
  on 
  rock 
  formations 
  of 
  comparable 
  

   resistance. 
  On 
  the 
  seaward 
  side 
  of 
  a 
  region 
  evidence 
  of 
  

   marine 
  denudation 
  must 
  be 
  expected 
  during 
  submergent 
  

   phases 
  and 
  such 
  may 
  well 
  be 
  less 
  noticeable 
  over 
  

   drowned 
  interior 
  lowlands, 
  as 
  at 
  present 
  illustrated 
  by 
  

   Chesapeake 
  and 
  Delaware 
  Bays. 
  Thus 
  not 
  only 
  must 
  

   the 
  evidence 
  of 
  successive 
  baselevels 
  be 
  sought, 
  but 
  the 
  

   fluvial 
  and 
  marine 
  phases 
  must 
  be 
  discriminated 
  from 
  

   each 
  other. 
  

  

  The 
  oldest 
  surface 
  which 
  concerns 
  the 
  present 
  investi- 
  

   gation 
  is 
  the 
  floor 
  on 
  which 
  the 
  Potomac 
  formations 
  were 
  

   laid 
  down. 
  Deposition 
  extended 
  intermittently 
  through 
  

   the 
  whole 
  of 
  the 
  Comanche 
  period 
  and 
  four 
  distinct 
  for- 
  

   mations 
  separated 
  by 
  unconformities 
  are 
  recognized 
  

   between 
  the 
  Potomac 
  and 
  Hudson 
  rivers. 
  These 
  are, 
  

   beginning 
  with 
  the 
  oldest, 
  the 
  Patuxent, 
  Arundel, 
  Pa- 
  

   tapsco, 
  and 
  Earitan. 
  All 
  are 
  continental 
  deposits, 
  al- 
  

   though 
  the 
  Earitan 
  shows 
  a 
  transition 
  toward 
  the 
  marine 
  

   conditions 
  of 
  the 
  Cretaceous. 
  

  

  [The 
  Earitan 
  formation 
  properly 
  belongs 
  in 
  the 
  Cre- 
  

   taceous 
  rather 
  than 
  Comanche, 
  but 
  for 
  the 
  .purpose 
  of 
  

   argument 
  as 
  to 
  the 
  complexity 
  of 
  the 
  physiographic 
  

   record 
  it 
  may 
  be 
  allowed 
  to 
  stand 
  as 
  given. 
  The 
  Eari- 
  

   tan 
  is 
  separated 
  from 
  both 
  the 
  underlying 
  and 
  overlying 
  

   formations 
  by 
  evident 
  unconformities, 
  whereas 
  the 
  over- 
  

   lying 
  Cretaceous 
  formations 
  are 
  apparently 
  conformable 
  

   among 
  themselves. 
  — 
  Editor.] 
  

  

  The 
  oldest 
  of 
  these 
  formations, 
  the 
  Patuxent, 
  rests 
  on 
  

  

  Am. 
  Jour. 
  Sci. 
  — 
  Fourth 
  Series, 
  Vol. 
  XLIX, 
  No. 
  294. 
  — 
  June, 
  1920. 
  

   29 
  

  

  