﻿408 
  J. 
  Barrell 
  — 
  The 
  Piedmont 
  Terraces 
  

  

  the 
  crystalline 
  rocks 
  at 
  "Washington, 
  whereas 
  the 
  young- 
  

   est; 
  the 
  Raritan, 
  forms 
  the 
  base 
  in 
  New 
  Jersey. 
  The 
  

   surface 
  of 
  unconformity 
  between 
  the 
  Patuxent 
  and 
  

   Arundel 
  at 
  Washington 
  must 
  come 
  to 
  be, 
  therefore, 
  

   between 
  the 
  crystallines 
  and 
  the 
  Arundel 
  at 
  some 
  locality 
  

   farther 
  northeast. 
  Likewise, 
  the 
  unconformities 
  at 
  the 
  

   base 
  of 
  the 
  Patapsco 
  and 
  Raritan 
  in 
  turn 
  bevel 
  across 
  

   the 
  older 
  erosion 
  surfaces 
  and 
  become 
  themselves 
  basal. 
  

   The 
  base 
  of 
  the 
  Potomac 
  group 
  is 
  thus 
  represented 
  by 
  

   four 
  distinct 
  erosion 
  surfaces 
  spaced 
  through 
  the 
  Co- 
  

   manche 
  period. 
  At 
  the 
  very 
  beginning 
  of 
  the 
  later 
  ero- 
  

   sion 
  history 
  of 
  the 
  Appalachians 
  the 
  problem 
  is 
  already 
  

   complex. 
  

  

  As 
  all 
  the 
  Potomac 
  formations 
  are 
  river 
  deposits 
  it 
  

   follows 
  that 
  the 
  surfaces 
  below 
  are 
  subaerial 
  peneplanes. 
  

   This 
  is 
  seen 
  in 
  the 
  deep 
  decay 
  of 
  the 
  crystalline 
  rocks 
  

   where 
  they 
  pass 
  below 
  the 
  overlying 
  formations. 
  More 
  

   convincing 
  still 
  is 
  the 
  stream 
  channeling 
  of 
  the 
  floors 
  ; 
  

   it 
  appears 
  from 
  the 
  figures 
  given 
  in 
  the 
  Washington 
  folio 
  

   that 
  these 
  old 
  valleys 
  are 
  one 
  to 
  three 
  miles 
  wide 
  and 
  

   about 
  one 
  hundred 
  feet 
  deep. 
  The 
  Arundel 
  formation, 
  

   where 
  it 
  outcrops, 
  is 
  recognized 
  as 
  lying 
  within 
  these 
  val- 
  

   leys 
  which 
  had 
  been 
  eroded 
  in 
  the 
  Patuxent. 
  Nearer 
  the 
  

   headwaters 
  of 
  the 
  streams 
  these 
  old 
  valleys 
  must 
  have 
  

   been 
  cut 
  in 
  the 
  crystalline 
  floor. 
  

  

  The 
  base 
  of 
  the 
  Raritan 
  is 
  well 
  exposed 
  in 
  New 
  Jersey 
  

   near 
  Staten 
  Island, 
  as 
  shown 
  on 
  the 
  geologic 
  map 
  of 
  the 
  

   Passaic 
  folio. 
  The 
  strike 
  of 
  the 
  formation, 
  where 
  it 
  

   rests 
  on 
  the 
  soft 
  Triassic 
  rocks, 
  is 
  N 
  47° 
  E 
  and 
  the 
  dip 
  

   is 
  45 
  feet 
  per 
  mile. 
  Where 
  the 
  formation 
  meets 
  the 
  

   resistant 
  serpentine 
  mass 
  of 
  Staten 
  Island 
  the 
  base 
  

   swings 
  nearly 
  four 
  miles 
  to 
  the 
  east. 
  There 
  is 
  no 
  evi- 
  

   dence 
  of 
  post-Raritan 
  faulting 
  or 
  warping; 
  the 
  serpen- 
  

   tine 
  still 
  rises 
  over 
  three 
  hundred 
  feet 
  above 
  sea-level. 
  

   Thus 
  at 
  the 
  time 
  of 
  the 
  Raritan 
  deposition, 
  the 
  serpen- 
  

   tine 
  mass 
  rose 
  as 
  a 
  monadnock, 
  probably 
  over 
  four 
  hun- 
  

   dred 
  feet 
  high, 
  above 
  a 
  peneplain 
  developed 
  on 
  the 
  soft 
  

   Triassic 
  formation. 
  

  

  The 
  floor 
  of 
  the 
  Potomac 
  group, 
  therefore, 
  consists 
  of 
  

   erosion 
  surfaces 
  of 
  fluvial 
  origin, 
  as 
  shown 
  by 
  the 
  nature 
  

   of 
  the 
  cover 
  and 
  the 
  weathering 
  and 
  topography 
  of 
  the 
  

   rock 
  floor. 
  

  

  All 
  the 
  Coastal 
  Plain 
  formations 
  of 
  the 
  Cretaceous 
  and 
  

   Tertiary 
  periods 
  are 
  marine 
  and 
  many 
  are 
  separated 
  by 
  

  

  