﻿GEOLOGY 
  OF 
  THE 
  PORT 
  LEYDEN 
  QUADRANGLE 
  49 
  

  

  that 
  the 
  stream 
  has 
  not 
  yet 
  cut 
  its 
  way 
  through 
  the 
  alluvium 
  and 
  

   reworked 
  drift 
  filling 
  the 
  valley 
  bottom, 
  and 
  also 
  because 
  glacial 
  

   striae 
  and 
  kames 
  near 
  the 
  river 
  level 
  have 
  not 
  been 
  disturbed. 
  Thus 
  

   also 
  the 
  slight 
  trench 
  cut 
  into 
  the 
  Precambrics 
  along 
  here 
  could 
  not 
  

   have 
  been 
  postglacial 
  in 
  origin. 
  

  

  There 
  is 
  still 
  the 
  possibility 
  that 
  glacial 
  waters 
  might 
  have 
  de- 
  

   veloped 
  the 
  terraces, 
  but 
  there 
  is 
  no 
  evidence 
  of 
  any 
  such 
  vigorous 
  

   water 
  action 
  especially 
  along 
  the 
  higher 
  part 
  of 
  the 
  limestone 
  ter- 
  

   race 
  where 
  records 
  would 
  surely 
  be 
  left. 
  Even 
  if 
  a 
  large 
  stream 
  had 
  

   flowed 
  along 
  the 
  ice 
  edge 
  and 
  under 
  the 
  steep 
  front 
  of 
  Tug 
  hill 
  its 
  

   gradient 
  w^ould 
  have 
  been 
  too 
  low 
  to 
  be 
  compatible 
  with 
  much 
  cut- 
  

   ting 
  power. 
  No 
  doubt 
  there 
  was 
  movement 
  of 
  water 
  along 
  the 
  wan- 
  

   ing 
  Black 
  river 
  ice 
  lobe, 
  but 
  the 
  only 
  current 
  of 
  any 
  importance 
  was 
  

   a 
  northerly 
  one 
  between 
  the 
  eastern 
  edge 
  of 
  the 
  limestone 
  terrace 
  

   and 
  the 
  ice 
  margin 
  [see 
  below]. 
  The 
  limestones 
  here 
  are 
  some- 
  

   what 
  water-worn, 
  but 
  the 
  stream 
  was 
  about 
  200 
  feet 
  below 
  the 
  top 
  

   of 
  the 
  terrace 
  and 
  thus 
  clearly 
  could 
  not 
  have 
  done 
  the 
  work 
  of 
  

   erosion 
  over 
  the 
  whole 
  terrace. 
  Also 
  the 
  presence 
  of 
  glacial 
  striae 
  

   on 
  the 
  terrace 
  shows 
  that 
  no 
  great 
  amount 
  of 
  erosion 
  could 
  have 
  

   taken 
  place 
  there 
  since 
  the 
  ice 
  retreat. 
  

  

  It 
  seems 
  certain 
  that 
  the 
  lowermost 
  Paleozoic 
  layers 
  must 
  havo 
  

   extended 
  farther 
  eastward, 
  by 
  overlap 
  on 
  the 
  Precambrics, 
  im- 
  

   mediately 
  preceding 
  the 
  glacial 
  period. 
  This 
  means 
  that 
  Black 
  river 
  

   was 
  some 
  distance 
  farther 
  eastward 
  and 
  that 
  the 
  western 
  tributaries 
  

   from 
  Tug 
  hill 
  entered 
  it 
  with 
  lower 
  gradients. 
  As 
  above 
  shown, 
  

   the 
  lowest 
  sedimentary 
  beds 
  could 
  not 
  have 
  been 
  cut 
  back 
  to 
  form 
  

   the 
  steep 
  slope 
  now 
  facing 
  Black 
  river 
  in 
  pre- 
  or 
  postglacial 
  times 
  

   nor 
  were 
  they 
  cut 
  back 
  by 
  glacial 
  waters. 
  Evidently 
  they 
  were 
  cut 
  

   back 
  by 
  the 
  ice 
  to 
  develop 
  the 
  steep 
  slope. 
  This 
  allowed 
  Black 
  

   river 
  to 
  shift 
  westward 
  to 
  its 
  present 
  position. 
  Thus 
  the 
  slight 
  

   trench 
  in 
  the 
  Precambrics 
  here 
  could 
  not 
  have 
  been 
  preglacial. 
  As 
  

   already 
  shown 
  it 
  is 
  clearly 
  not 
  postglacial 
  and 
  apparently 
  it 
  was 
  

   formed 
  by 
  ice 
  cutting. 
  The 
  concave 
  character 
  of 
  this 
  inner 
  por- 
  

   tion 
  of 
  the 
  valley 
  is 
  brought 
  out 
  in 
  the 
  figure 
  and 
  strongly 
  suggests 
  

   ice 
  work. 
  

  

  The 
  fact 
  should 
  also 
  be 
  considered 
  that 
  we 
  are 
  here 
  dealing 
  with 
  

   unaltered 
  sedimentaries, 
  with 
  slightly 
  upturned 
  edges, 
  resting 
  upon 
  

   a 
  rather 
  smooth 
  surface 
  of 
  igneous 
  and 
  metamorphic 
  rocks, 
  and 
  that 
  

   the 
  lowest 
  sediments 
  are 
  weak 
  sandstones 
  and 
  sandy 
  limestones, 
  

   which 
  greatly 
  favored 
  the 
  stripping 
  off 
  power 
  of 
  the 
  ice. 
  Robert 
  

  

  