﻿328 
  J. 
  Barrell 
  — 
  The 
  Piedmont 
  Terraces 
  

  

  plished 
  in 
  the 
  Comanche 
  period. 
  The 
  base 
  of 
  the 
  oldest 
  

   Comanche 
  formation 
  at 
  Washington 
  has 
  the 
  relatively 
  

   steep 
  slope 
  of 
  115 
  feet 
  per 
  mile 
  ; 
  at 
  Baltimore 
  the 
  slope 
  

   is 
  94 
  feet 
  per 
  mile. 
  The 
  base 
  of 
  the 
  Cretaceous 
  has 
  a 
  

   mnch 
  flatter 
  slope 
  at 
  both 
  places, 
  namely, 
  33 
  feet 
  per 
  mile. 
  

  

  Eecent 
  study 
  of 
  the 
  Coastal 
  Plain 
  formations 
  has 
  re- 
  

   vealed 
  what 
  to 
  older 
  geologists 
  would 
  have 
  seemed 
  a 
  

   surprising 
  number 
  of 
  unconformities. 
  In 
  the 
  Patuxent 
  

   quadrangle 
  just 
  west 
  of 
  Washington, 
  for 
  instance, 
  thir- 
  

   teen 
  unconformities 
  have 
  been 
  detected 
  between 
  the 
  

   crystalline 
  rocks 
  of 
  pre-Comanche 
  age 
  and 
  the 
  present 
  

   surface. 
  15 
  In 
  fact, 
  there 
  are 
  only 
  two 
  surfaces 
  between 
  

   formations 
  that 
  are 
  not 
  mapped 
  as 
  unconformities. 
  

   Some 
  of 
  these 
  unconformities 
  represent 
  very 
  long 
  

   periods 
  of 
  time, 
  as 
  shown 
  by 
  marked 
  change 
  in 
  tilt 
  

   between 
  formations, 
  the 
  great 
  volume 
  of 
  formations, 
  and 
  

   marked 
  change 
  in 
  character 
  of 
  sediments 
  ; 
  most 
  impor- 
  

   tant, 
  however, 
  by 
  marked 
  changes 
  in 
  faunas. 
  Some 
  of 
  

   the 
  time 
  intervals 
  are 
  represented 
  in 
  other 
  parts 
  of 
  the 
  

   Coastal 
  Plain 
  by 
  formations 
  which 
  are 
  absent 
  here, 
  but 
  

   even 
  these 
  additions 
  do 
  not 
  begin 
  to 
  make 
  a 
  complete 
  

   record. 
  In 
  the 
  opinion 
  of 
  the 
  writer 
  much 
  less 
  than 
  half 
  

   of 
  post-Jurassic 
  time 
  is 
  represented 
  by 
  actual 
  sediments 
  

   in 
  the 
  Patuxent 
  quadrangle. 
  An 
  especially 
  long 
  interval 
  

   at 
  the 
  close 
  of 
  the 
  Cretaceous 
  appears 
  to 
  be 
  unrepre- 
  

   sented, 
  the 
  entire 
  Oligocene 
  is 
  missing, 
  and 
  there 
  is 
  

   another 
  long 
  gap 
  between 
  the 
  Miocene 
  and 
  the 
  late 
  Plio- 
  

   cene 
  formations. 
  

  

  Each 
  of 
  these 
  unconformities 
  marks 
  a 
  change 
  of 
  base- 
  

   level 
  that 
  must 
  have 
  affected 
  the 
  Piedmont 
  Plateau 
  as 
  

   well 
  as 
  the 
  Coastal 
  Plain. 
  Some 
  of 
  the 
  erosion 
  intervals 
  

   were 
  so 
  prolonged 
  that 
  the 
  Plateau 
  must 
  have 
  become 
  

   peneplaned 
  with 
  respect 
  to 
  the 
  new 
  baselevel.^ 
  In 
  view 
  

   of 
  this 
  evidence 
  in 
  the 
  Coastal 
  Plain 
  of 
  intermittent 
  tilt- 
  

   ing 
  and 
  uplift 
  of 
  the 
  Piedmont 
  and 
  Appalachian 
  region, 
  

   how 
  can 
  the 
  post-Jurassic 
  erosion 
  history 
  of 
  that 
  region 
  

   be 
  interpreted 
  in 
  terms 
  of 
  one 
  Jurassic, 
  one 
  Cretaceous, 
  

   one 
  early 
  Tertiary, 
  and 
  one 
  late 
  Tertiary 
  baselevel? 
  

   Even 
  if 
  four 
  dominant 
  baselevels 
  have 
  been 
  determined, 
  

   the 
  -ages 
  assigned 
  to 
  them 
  are, 
  in 
  the 
  nature 
  of 
  things, 
  

   hardly 
  more 
  than 
  mere 
  guesses. 
  

  

  15 
  Columnar 
  section, 
  Patuxent 
  folio, 
  IT. 
  S. 
  G. 
  S. 
  1907. 
  

  

  