﻿GEOLOGY 
  OF 
  THE 
  PORT 
  LEYDEN 
  QUADRANGLE 
  47 
  

  

  Erosion 
  of 
  the 
  sedimentaries 
  

  

  Turning 
  our 
  attention 
  to 
  the 
  sedimentaries, 
  we 
  find 
  that 
  ice 
  erosion 
  

   was 
  much 
  more 
  effective 
  upon 
  them. 
  In 
  fact 
  the 
  writer 
  beheves 
  

   that 
  in. 
  the 
  Black 
  river 
  valley 
  we 
  have 
  one 
  of 
  the 
  best 
  examples 
  of 
  

   ice 
  erosion 
  in 
  northern 
  New 
  York. 
  One 
  factor 
  favoring 
  the 
  ice 
  

   work 
  here 
  was 
  the 
  comparative 
  softness 
  and 
  highly 
  jointed 
  char- 
  

   acter 
  of 
  the 
  rocks, 
  while 
  another 
  factor 
  was 
  their 
  position 
  with 
  ref- 
  

   erence 
  to 
  the 
  ice 
  current. 
  

  

  The 
  figure 
  on 
  page 
  48 
  shows 
  the 
  profile 
  and 
  geologic 
  structure 
  

   across 
  the 
  Black 
  river 
  valley 
  2^ 
  miles 
  north 
  of 
  Lyons 
  Falls. 
  One 
  

   of 
  the 
  striking 
  features 
  is 
  the 
  terraced 
  character 
  of 
  the 
  sedimentar- 
  

   ies, 
  particularly 
  from 
  Port 
  Leyden 
  northward 
  [see 
  topographic 
  

   map]. 
  Along 
  the 
  river 
  course 
  there 
  is 
  a 
  slight 
  notch 
  in 
  the 
  Pre- 
  

   cambrics 
  and 
  just 
  west 
  of 
  this, 
  on 
  the 
  northern 
  part 
  of 
  the 
  Port 
  

   Leyden 
  sheet, 
  there 
  is 
  a 
  steep 
  slope 
  rising 
  300 
  feet 
  above 
  the 
  Pre- 
  

   cambrics. 
  The 
  formations 
  outcropping 
  on 
  this 
  slope 
  are 
  shown 
  in 
  

   the 
  section. 
  Resting 
  upon 
  the 
  Precambrics 
  are 
  several 
  feet 
  of 
  weak 
  

   sandstones 
  which 
  are 
  followed 
  by 
  the 
  sandy 
  limestones 
  of 
  the 
  Pa- 
  

   melia 
  ; 
  then 
  come 
  the 
  hard 
  Lowville 
  and 
  Black 
  River 
  limestones 
  ; 
  

   while 
  the 
  summit 
  is 
  capped 
  by 
  hard, 
  crystalline 
  limestones. 
  The 
  

   streams 
  passing 
  over 
  this 
  slope 
  are 
  characterized 
  by 
  gorges 
  with 
  

   waterfalls 
  and 
  rapids. 
  From 
  the 
  summit 
  of 
  this 
  slope 
  and 
  extend- 
  

   ing 
  for 
  several 
  miles 
  westward 
  there 
  is 
  a 
  well 
  defined 
  terrace 
  

   developed 
  upon 
  the 
  limestone. 
  

  

  Rising 
  from 
  the 
  western 
  side 
  of 
  the 
  above 
  named 
  terrace 
  there 
  is 
  

   a 
  second 
  slope 
  higher 
  and 
  much 
  steeper 
  than 
  the 
  first. 
  The 
  rise 
  is 
  

   commonly 
  about 
  450 
  feet 
  within 
  Ys 
  mile. 
  The 
  soft 
  Utica 
  shales 
  

   outcrop 
  at 
  the 
  base 
  of 
  this 
  slope 
  and 
  they 
  are 
  followed 
  by 
  the 
  

   Lorraine 
  shales 
  with 
  an 
  upward 
  increasing 
  sandstone 
  content. 
  The 
  

   summit 
  of 
  this 
  terrace, 
  known 
  as 
  Tug 
  hill, 
  is 
  more 
  irregular 
  and 
  

   stream 
  dissected 
  than 
  the 
  limestone 
  terrace 
  below. 
  All 
  streams 
  

   flowing 
  across 
  the 
  steep 
  slope 
  of 
  this 
  terrace 
  have 
  high 
  gradients 
  

   and 
  have 
  cut 
  deep, 
  narrow 
  gorges 
  locally 
  called 
  '' 
  gulfs." 
  

  

  At 
  first 
  these 
  terraces, 
  in 
  their 
  present 
  form, 
  were 
  thought 
  to 
  have 
  

   been 
  due 
  entirely 
  to 
  water 
  action, 
  but 
  an 
  examination 
  of 
  the 
  region 
  

   shows 
  that 
  some 
  other 
  explanation 
  must 
  be 
  sought. 
  The 
  steep 
  

   fronts 
  of 
  the 
  terraces 
  are 
  certainly 
  young 
  topographic 
  features, 
  

   which 
  precludes 
  the 
  possibility 
  of 
  their 
  having 
  been 
  formed 
  during 
  

   the 
  long 
  preglacial 
  period 
  of 
  erosion 
  in 
  this 
  ancient 
  region. 
  On 
  the 
  

   other 
  hand. 
  Black 
  river 
  has 
  done 
  very 
  little 
  work 
  of 
  erosion, 
  between 
  

   Lyons 
  Falls 
  and 
  Lowville, 
  in 
  postglacial 
  times 
  as 
  proved 
  by 
  the 
  fact 
  

  

  