﻿146 
  . 
  NEW 
  YORK 
  STATE 
  MUSEUM 
  

  

  The 
  genesis 
  of 
  this 
  prevaihng 
  orientation 
  probably 
  involves 
  factors 
  

   which 
  cover 
  the 
  entire 
  geologic 
  history 
  of 
  the 
  region. 
  In 
  an 
  earlier 
  

   chapter 
  Professor 
  Gushing 
  has 
  shown 
  that 
  during 
  the 
  time 
  of 
  the 
  

   earliest 
  sedimentation 
  in 
  the 
  region 
  there 
  was 
  alternately 
  a 
  tipping 
  

   to 
  the 
  northeast 
  and 
  the 
  southwest, 
  the 
  fulcrum 
  of 
  motion 
  lying 
  

   across 
  our 
  district, 
  initiating 
  what 
  is 
  called 
  the 
  Frontenac 
  axis 
  

   [p- 
  95]- 
  The 
  broad 
  depression 
  of 
  the 
  valley 
  is 
  thought 
  to 
  be 
  partly 
  

   the 
  result 
  of 
  sagging, 
  accompanied 
  by 
  jointing, 
  one 
  main 
  trend 
  of 
  

   joints 
  having 
  fair 
  agreement 
  with 
  the 
  trend 
  of 
  the 
  valley. 
  

   Gushing 
  also 
  shows 
  that 
  some 
  slight 
  folding 
  occurred 
  in 
  Paleozoic 
  

   time 
  and 
  stronger 
  folding 
  in 
  Precambric 
  time 
  which 
  probably 
  had 
  

   some 
  directive 
  influence 
  on 
  the 
  drainage 
  [p. 
  108-T15]. 
  

  

  The 
  larger 
  existing 
  features 
  and 
  general 
  stream 
  directions 
  were 
  

   developed 
  during 
  Tertiary 
  time 
  under 
  subatmospheric 
  erosion. 
  Dur- 
  

   ing 
  Pleistocene 
  time 
  the 
  St 
  Lawrence 
  valley, 
  being 
  closely 
  in 
  line 
  

   with 
  the 
  spreading 
  flow 
  of 
  the 
  ice 
  sheet 
  over 
  the 
  region, 
  served 
  as 
  a 
  

   trough 
  for 
  the 
  advancing 
  and 
  the 
  waning 
  ice 
  lobes. 
  We 
  do 
  not 
  know 
  

   the 
  number 
  of 
  ice 
  invasions 
  but 
  it 
  seems 
  quite 
  certain 
  that 
  the 
  latest, 
  

   or 
  Wisconsin, 
  ice 
  sheet 
  was 
  preceded 
  by 
  others 
  of 
  probably 
  greatei 
  

   effectiveness 
  in 
  erosion. 
  The 
  striking 
  parallelism 
  of 
  the 
  minor 
  

   features 
  of 
  the 
  topography 
  is 
  probably 
  due 
  in 
  some 
  degree 
  to 
  re- 
  

   peated 
  glaciation, 
  the 
  alternation 
  of 
  ice 
  flow 
  of 
  the 
  glacial 
  epochs 
  

   and 
  the 
  stream 
  erosion 
  of 
  the 
  interglacial 
  epochs 
  mutually 
  assisting 
  

   or 
  guiding 
  each 
  other. 
  

  

  Dominant 
  types. 
  The 
  topographic 
  features 
  in 
  the 
  sedimentary 
  

   rocks 
  are 
  naturally 
  an 
  expression 
  largely 
  of 
  the 
  stratigraphic 
  char- 
  

   acters. 
  This 
  has 
  already 
  been 
  discussed 
  in 
  a 
  former 
  chapter 
  by 
  

   Gushing 
  [p. 
  121-136]. 
  In 
  the 
  present 
  connection 
  we 
  have 
  to 
  con- 
  

   sider 
  the 
  topography 
  in 
  its 
  relation 
  to 
  the 
  glacial 
  and 
  glacio- 
  

   aqueous 
  history. 
  

  

  Leaving 
  out 
  of 
  account 
  for 
  the 
  present 
  the 
  localized 
  and 
  scanty 
  

   moraine 
  deposits, 
  we 
  may 
  distinguish 
  two 
  dominant 
  types 
  of 
  the 
  

   surface 
  relief 
  in 
  the 
  area, 
  (i) 
  the 
  rounded 
  rock 
  hills 
  or 
  knobs 
  ol 
  

   rather 
  striking 
  relief 
  in 
  the 
  northern 
  part 
  of 
  the 
  area, 
  in 
  the 
  district 
  

   of 
  Potsdam 
  and 
  Precambric 
  rocks, 
  and 
  (2) 
  the 
  broad 
  level 
  stretche=^ 
  

   which 
  characterize 
  the 
  southern 
  half 
  of 
  the 
  area, 
  where 
  the 
  rocks 
  

   are 
  well 
  stratified. 
  

  

  Rock 
  knobs. 
  In 
  the 
  northern 
  part 
  of 
  the 
  area, 
  covered 
  by 
  the 
  

   Grindstone 
  and 
  Alexandria 
  sheets 
  and 
  the 
  northeast 
  part 
  of 
  the 
  

   Theresa 
  sheet, 
  the 
  crystalline 
  rocks 
  and 
  the 
  lower 
  Potsdam 
  appear 
  

   commonly 
  in 
  the 
  form 
  of 
  knobs 
  or 
  bosses, 
  singly 
  or 
  in 
  clusters 
  and 
  

   chains, 
  as 
  illustrated 
  in 
  plates 
  6 
  and 
  7. 
  Gushing 
  has 
  shown 
  [p. 
  54] 
  

  

  