﻿of 
  the 
  Northern 
  Appalachians. 
  421 
  

  

  Piedmont 
  and 
  South 
  Mountains, 
  as 
  shown 
  by 
  the 
  upper 
  

   figure 
  in 
  PL 
  VI. 
  The 
  restored 
  upper 
  surface 
  of 
  the 
  Cre- 
  

   taceous 
  deposits 
  is 
  shown 
  by 
  the 
  curved 
  line 
  which 
  passes 
  

   slightly 
  above 
  the 
  summits 
  of 
  the 
  South 
  Mountains 
  and 
  

   beneath 
  the 
  present 
  surface 
  of 
  the 
  country 
  about 
  10 
  miles 
  

   southeast 
  of 
  Washington. 
  A 
  note 
  on 
  the 
  drawing 
  indi- 
  

   cates 
  that 
  the 
  slope 
  of 
  this 
  surface 
  in 
  the 
  vicinity 
  of 
  

   Washington 
  was 
  to 
  be 
  increased 
  from 
  29 
  to 
  31 
  degrees. 
  

   The 
  effect 
  of 
  this 
  at 
  E 
  10 
  miles 
  would 
  be 
  to 
  place 
  the 
  sur- 
  

   face 
  170 
  feet 
  instead 
  of 
  300 
  feet 
  above 
  sea-level, 
  making 
  

   the 
  estimated 
  original 
  thickness 
  of 
  the 
  formation 
  at 
  this 
  

   point 
  150 
  as 
  against 
  280 
  feet. 
  Also 
  the 
  surface 
  would 
  

   meet 
  the 
  base 
  of 
  the 
  Eocene 
  formation 
  at 
  50 
  to 
  60 
  miles 
  

   instead 
  of 
  at 
  80 
  miles 
  east 
  of 
  Washington, 
  thus 
  consider- 
  

   ably 
  reducing 
  the 
  distance 
  through 
  which 
  the 
  Cretaceous 
  

   formations 
  were 
  beveled 
  in 
  the 
  succeeding 
  erosion 
  cycle. 
  

  

  The 
  restored 
  surface 
  is 
  shown 
  as 
  lying 
  less 
  than 
  100 
  

   feet 
  above 
  the 
  highest 
  summits 
  of 
  South 
  Mountains 
  and 
  

   as 
  below 
  one 
  peak 
  (Quirauk 
  Mt., 
  Md.). 
  As 
  it 
  represents 
  

   the 
  upper 
  surface 
  of 
  the 
  Cretaceous 
  deposits 
  it 
  will 
  be 
  

   seen 
  that 
  post-Cretaceous 
  erosion 
  is 
  assumed 
  to 
  have 
  left 
  

   a 
  considerable 
  number 
  of 
  practically 
  unreduced 
  Creta- 
  

   ceous 
  residuals. 
  "The 
  fair 
  degree 
  of 
  preservation 
  of 
  

   that 
  [Cretaceous] 
  plain 
  upon 
  resistant 
  rocks 
  and 
  the 
  

   present 
  broadly 
  mountainous 
  character 
  of 
  the 
  Appa- 
  

   lachians 
  are 
  due 
  largely 
  to 
  the 
  comparative 
  recency 
  of 
  

   the 
  last 
  strong 
  upward 
  movements, 
  which 
  appear 
  to 
  date 
  

   from 
  the 
  close 
  of 
  the 
  Miocene. 
  " 
  36 
  

  

  The 
  linear 
  projection 
  of 
  the 
  base 
  of 
  the 
  Eocene 
  passes 
  

   over 
  the 
  Piedmont 
  and 
  meets 
  the 
  eastern 
  slope 
  of 
  the 
  

   South 
  Mountains 
  at 
  an 
  elevation 
  of 
  1,400 
  feet. 
  The 
  base 
  

   would 
  pass 
  over 
  the 
  Piedmont 
  even 
  if 
  it 
  were 
  bent 
  down 
  

   in 
  a 
  manner 
  similar 
  to 
  the 
  restored 
  Cretaceous 
  surface. 
  

   It 
  would 
  appear 
  quite 
  possible, 
  therefore, 
  that 
  the 
  sea 
  

   might 
  have 
  covered 
  the 
  present 
  Piedmont 
  ' 
  surface 
  in 
  

   whole 
  or 
  in 
  part 
  during 
  the 
  Eocene. 
  Professor 
  Barrell 
  

   considered, 
  however, 
  that 
  the 
  Cretaceous 
  terraces 
  in 
  New 
  

   England 
  had 
  experienced 
  a 
  notably 
  greater 
  dissection 
  

   than 
  those 
  at 
  lower 
  elevations, 
  which 
  indicated 
  a 
  long 
  

   time 
  interval, 
  and 
  consequently 
  he 
  assumed 
  that 
  during 
  

   the 
  Eocene 
  there 
  had 
  been 
  "emergence 
  of 
  several 
  hun- 
  

   dred 
  feet 
  and 
  retreat 
  of 
  the 
  shore 
  to 
  the 
  region 
  of 
  the 
  

   present 
  Coastal 
  Plain.' 
  ' 
  As 
  a 
  result 
  it 
  must 
  be 
  sup- 
  

  

  86 
  Loc. 
  eit., 
  p. 
  105. 
  

  

  