﻿no 
  Geological 
  Survey 
  of 
  Louisiana 
  [Sect. 
  

  

  tion 
  of 
  sand 
  and 
  gravel 
  in 
  the 
  main 
  valley 
  would 
  cease 
  and 
  their 
  

   place 
  be 
  taken 
  by 
  fine 
  cypress 
  swamp 
  clays. 
  In 
  these 
  cypress 
  

   swamp 
  clays 
  local 
  beds 
  of 
  sand 
  and 
  loam 
  were 
  formed 
  along 
  the 
  

   sluggish 
  streams 
  which 
  meander 
  aimlessly 
  through 
  the 
  valley. 
  

  

  In 
  the 
  Mississippi 
  valley 
  the 
  irregular 
  melting 
  of 
  the 
  glaciers 
  

   which 
  occupied 
  the 
  whole 
  region 
  north 
  of 
  the 
  Ohio 
  and 
  Missouri 
  

   rivers 
  caused 
  great 
  floods 
  which 
  brought 
  with 
  them 
  large 
  quan- 
  

   tities 
  of 
  glacial 
  rock 
  meal. 
  At 
  times 
  this 
  flood 
  may 
  even 
  have 
  

   overflowed 
  the 
  bounding 
  hills 
  or 
  bluffs 
  of 
  the 
  old 
  valley 
  and 
  

   formed 
  on 
  their 
  summits 
  great 
  natural 
  levees 
  of 
  silt 
  even 
  as 
  the 
  

   pigmy 
  Mississippi 
  does 
  to-day. 
  At 
  any 
  rate 
  these 
  periodical 
  

   floods, 
  caused 
  by 
  variations 
  in 
  temperature 
  along 
  the 
  ice 
  front, 
  

   must 
  have 
  formed 
  extensive 
  mud 
  flats, 
  as 
  wide 
  as 
  the 
  river 
  

   valley, 
  and 
  winds 
  blowing 
  over 
  them 
  would 
  experience 
  no 
  diffi- 
  

   culty 
  in 
  transporting 
  this 
  impalpable 
  silt 
  to 
  the 
  summits 
  of 
  the 
  

   bordering 
  hills. 
  

  

  Period 
  of 
  elevatioji. 
  — 
  At 
  the 
  close 
  of 
  this 
  subsidence 
  in 
  which 
  

   the 
  land 
  reached 
  a 
  level 
  a 
  hundred 
  feet* 
  lower 
  than 
  to-day, 
  a 
  

   period 
  of 
  elevation 
  commenced. 
  During 
  this 
  time 
  an 
  eleva- 
  

   tion 
  slightly 
  above 
  the 
  present 
  was 
  reached 
  and 
  the 
  river 
  

   cut 
  out 
  the 
  deposits 
  of 
  the 
  preceding 
  period. 
  The 
  amount 
  

   of 
  this 
  excavation 
  can 
  be 
  judged 
  by 
  the 
  height 
  of 
  the 
  Port 
  Hud- 
  

   son 
  bluffs 
  and 
  the 
  Opelousas, 
  Carrencro 
  and 
  Cte 
  Gelee 
  hills. 
  

   To 
  this 
  is 
  to 
  be 
  added 
  the 
  very 
  inconsiderable 
  depth 
  of 
  the 
  older 
  

   material 
  below 
  the 
  present 
  alluvium. 
  In 
  the 
  upper 
  Red 
  river 
  

   valley 
  this 
  excavation 
  amounted 
  to 
  about 
  60 
  feet. 
  

  

  Present 
  period 
  of 
  siibsidence. 
  — 
  At 
  the 
  close 
  of 
  this 
  elevation 
  the 
  

   present 
  period 
  of 
  subsidence 
  commenced 
  and 
  wdth 
  it 
  the 
  deposi- 
  

   tion 
  of 
  the 
  alluvium. 
  That 
  a 
  subsidence 
  is 
  going 
  on 
  is 
  

   evidenced 
  by 
  a 
  number 
  of 
  facts 
  : 
  (i) 
  by 
  the 
  drowned 
  condition 
  

   of 
  the 
  mouths 
  of 
  the 
  majority 
  of 
  coastal 
  rivers 
  ; 
  (2) 
  by 
  the 
  

  

  *If 
  the 
  yellow 
  loam 
  and 
  the 
  Icess 
  are 
  not 
  considered, 
  a 
  subsidence 
  of 
  this 
  

   amount 
  seems 
  to 
  be 
  quite 
  capable 
  of 
  producing 
  the 
  deposits 
  observed 
  in 
  the 
  

   Mississippi 
  valley. 
  The 
  origin 
  of 
  the 
  loess 
  is 
  so 
  little 
  understood 
  that 
  an 
  

   assumption 
  that 
  the 
  subsidence 
  was 
  equal 
  to 
  the 
  height 
  of. 
  the 
  highest 
  loess 
  

   above 
  sea 
  level 
  seems 
  hardly 
  well 
  founded. 
  If 
  the 
  subsidence 
  was 
  so 
  great, 
  

   about 
  500 
  feet, 
  as 
  the 
  estuarian 
  theory 
  of 
  the 
  origin 
  of 
  loess 
  demands, 
  we 
  

   should 
  find 
  well 
  marked 
  marine 
  forms 
  at 
  Baton 
  Rouge 
  and 
  Port 
  Hudson. 
  

  

  