﻿NIAGARA 
  FALLS 
  AND 
  VICINITY 
  57 
  

  

  Throughout 
  the 
  greater 
  part 
  of 
  the 
  district, 
  the 
  poHshed 
  rock 
  sur- 
  

   faces 
  are 
  covered 
  by 
  a 
  coating 
  of 
  drift 
  of 
  very 
  varying 
  character 
  

   and 
  thickness. 
  This 
  was 
  the 
  ground 
  moraine 
  or 
  till 
  of 
  the 
  Lauren- 
  

   tian 
  glacier, 
  and 
  represents 
  the 
  rock 
  debris 
  which 
  was 
  frozen 
  into 
  

   the 
  bottom 
  of 
  the 
  ice, 
  and 
  carried 
  along 
  in 
  its 
  motion, 
  till 
  liberated 
  

   by 
  the 
  melting 
  of 
  the 
  ice. 
  This 
  ground 
  moraine, 
  either 
  in 
  its 
  

   original 
  heterogeneous 
  character 
  or 
  modified 
  by 
  the 
  agency 
  of 
  

   running 
  water, 
  filled 
  most 
  of 
  the 
  old 
  river 
  gorges 
  through 
  which 
  the 
  

   drainage 
  of 
  preglacial 
  times 
  found 
  its 
  exit. 
  Some 
  of 
  the 
  shallower 
  

   lowlands, 
  like 
  that 
  of 
  the 
  Tonawanda, 
  were 
  also 
  filled 
  with 
  drift, 
  

   while 
  the 
  more 
  profound 
  ones, 
  like 
  the 
  Erie 
  and 
  Ontario 
  lowlands, 
  

   received 
  only 
  a 
  partial 
  drift 
  filling. 
  

  

  The 
  partial 
  obliteration 
  of 
  the 
  old 
  drainage 
  channels, 
  which 
  wai 
  

   thus 
  brought 
  about, 
  together 
  with 
  a 
  depression 
  of 
  the 
  land 
  on 
  the 
  

   northeast 
  to 
  a 
  depth 
  below 
  that 
  at 
  which 
  it 
  now 
  stands, 
  converted 
  

   the 
  unfilled 
  lowlands 
  into 
  lake 
  basins, 
  apparently 
  reversed 
  the 
  drain- 
  

   age 
  of 
  many 
  streams, 
  forcing 
  them 
  to 
  cut 
  gorges 
  where 
  their 
  old 
  

   channels 
  were 
  drift-filled, 
  and 
  finally 
  became 
  the 
  immediate 
  factors 
  

   in 
  the 
  formation 
  of 
  Niagara. 
  

  

  Lacustrine 
  period^ 
  

  

  During 
  the 
  slow 
  melting 
  of 
  the 
  glaciers 
  in 
  the 
  Laurentian 
  region, 
  

   and 
  the 
  resultant 
  northward 
  retreat 
  of 
  the 
  front 
  of 
  the 
  ice, 
  large 
  

   bodies 
  of 
  water, 
  of 
  varying 
  depth 
  and 
  extent, 
  were 
  held 
  in 
  front 
  of 
  

   the 
  ice 
  sheet, 
  which 
  formed 
  a 
  dam 
  across 
  the 
  northeastern 
  part 
  of 
  

   the 
  lowland 
  country, 
  the 
  general 
  slope 
  of 
  which 
  was 
  now 
  toward 
  

   the 
  ice 
  instead 
  of 
  away 
  from 
  it. 
  The 
  elevations 
  of 
  these 
  glacial 
  

   lakes 
  were 
  determined 
  by 
  the 
  lowest 
  uncovered 
  passes 
  in 
  the 
  

   margins 
  of 
  the 
  lake 
  basins 
  across 
  which 
  the 
  discharge 
  took 
  place, 
  

   and, 
  as 
  during 
  the 
  continued 
  melting 
  of 
  the 
  ice 
  dam, 
  lower 
  passes 
  

   were 
  progressively 
  uncovered, 
  the 
  outlets 
  were 
  successively 
  trans- 
  

   ferred 
  to 
  them 
  and 
  the 
  levels 
  of 
  the 
  lakes 
  sank 
  correspondingly. 
  

  

  ^For 
  a 
  detailed 
  account 
  of 
  the 
  successive 
  stages 
  in 
  the 
  development 
  of 
  

   the 
  great 
  lakes, 
  the 
  shore 
  lines, 
  outlets 
  and 
  extent 
  of 
  each, 
  the 
  reader 
  is 
  

   referred 
  to 
  the 
  papers 
  by 
  Gilbert, 
  Spencer, 
  Taylor, 
  Leverett, 
  Fairchild 
  and 
  

   others, 
  cited 
  in 
  the 
  appendix. 
  

  

  