﻿of 
  the 
  Northern 
  Appalachians. 
  409 
  

  

  unconformities. 
  The 
  basal 
  beds 
  must 
  rest 
  on 
  an 
  erosion 
  

   surface 
  which 
  was 
  presumably 
  subaerial 
  but 
  was 
  modi- 
  

   fied 
  by 
  marine 
  action. 
  North 
  of 
  the 
  Rappahannock 
  

   Eiver 
  in 
  Virginia 
  these 
  formations, 
  however, 
  rest 
  on 
  

   older 
  unconsolidated 
  deposits. 
  The 
  contacts 
  are 
  charac- 
  

   teristically 
  plane 
  surfaces 
  but 
  difficult 
  to 
  trace 
  in 
  detail. 
  

   There 
  is 
  evidence 
  near 
  Richmond 
  and 
  Petersburg 
  that 
  

   the 
  Eocene 
  beds 
  were 
  laid 
  down 
  on 
  an 
  irregularly 
  eroded 
  

   surface 
  having 
  almost 
  as 
  much 
  relief 
  as 
  the 
  present 
  

   topography. 
  The 
  evidence 
  consists 
  of 
  inliers 
  of 
  Pa- 
  

   munky 
  (Eocene) 
  among 
  hills 
  of 
  Potomac 
  material. 
  31 
  

  

  South 
  of 
  the 
  Rappahannock 
  the 
  Miocene 
  beds 
  overlap 
  

   the 
  older 
  Coastal 
  Plain 
  deposits 
  and 
  rest 
  on 
  the 
  crystal- 
  

   line 
  rocks 
  of 
  the 
  Piedmont 
  Plateau. 
  The 
  character 
  of 
  

   this 
  contact 
  should 
  be 
  carefully 
  studied. 
  Judged 
  from 
  

   the 
  geologic 
  and 
  topographic 
  maps 
  it 
  is 
  a 
  very 
  uniform 
  

   plane 
  surface. 
  The 
  beds 
  have 
  an 
  average 
  dip 
  of 
  ten 
  feet 
  

   per 
  mile. 
  The 
  slope 
  of 
  the 
  floor 
  from 
  which 
  the 
  Miocene 
  

   has 
  been 
  stripped 
  may 
  be 
  steeper 
  west 
  of 
  the 
  outcrop, 
  

   indicating 
  a 
  westward 
  advance 
  of 
  the 
  sea 
  during 
  the 
  de- 
  

   position 
  of 
  the 
  Calvert, 
  which 
  forms 
  the 
  lower 
  part 
  of 
  

   the 
  Miocene. 
  There 
  is 
  no 
  indication 
  in 
  this 
  locality 
  

   (Goochland 
  and 
  Amelia 
  quadrangles) 
  of 
  the 
  landward 
  

   limits 
  of 
  the 
  later 
  Miocene 
  formations, 
  the 
  St. 
  Mary's 
  

   and 
  Yorktown, 
  as 
  the 
  extension 
  of 
  these 
  at 
  a 
  slope 
  of 
  ten 
  

   feet 
  per 
  mile 
  passes 
  above 
  the 
  present 
  surface 
  of 
  the 
  

   Piedmont 
  Plateau 
  for 
  a 
  distance 
  of 
  more 
  than 
  eighty 
  

   miles. 
  

  

  The 
  Fall 
  Line 
  and 
  Relation 
  op 
  Marine 
  Planation 
  to 
  Geologic 
  

  

  Formations. 
  

  

  The 
  shore-line 
  of 
  southern 
  New 
  England 
  departs 
  

   somewhat 
  from 
  the 
  northward 
  extension 
  of 
  the 
  fall 
  line 
  

   which 
  separates 
  the 
  Coastal 
  Plain 
  and 
  the 
  Piedmont 
  

   Plateau 
  and 
  cuts 
  across 
  the 
  geologic 
  formations 
  to 
  an 
  

   exceptional 
  degree. 
  During 
  the 
  later 
  Mesozoic 
  and 
  Ter- 
  

   tiary, 
  however, 
  the 
  shore-line 
  must 
  have 
  been 
  much 
  more 
  

   nearly 
  parallel 
  to 
  the. 
  fall 
  line 
  throughout 
  its 
  length 
  and 
  

   thus 
  less 
  markedly 
  oblique 
  to 
  the 
  older 
  mountain 
  struc- 
  

   tures, 
  as 
  indicated 
  in 
  fig. 
  1. 
  

  

  The 
  fall 
  line 
  is 
  merely 
  the 
  belt 
  of 
  recently 
  exposed 
  

   floor 
  at 
  the 
  inner 
  margin 
  of 
  the 
  Coastal 
  Plain. 
  This 
  

  

  31 
  Samuel 
  Sanf 
  ord. 
  Underground 
  Water 
  Eesources 
  of 
  the 
  Coastal 
  Plain 
  

   Province 
  of 
  Virginia. 
  Va. 
  Geol. 
  Survey, 
  Bull. 
  No. 
  5, 
  1913, 
  pp. 
  17-18. 
  

  

  