﻿6 
  MUSSELS 
  OF 
  CENTRAL 
  AND 
  NORTHERN 
  MINNESOTA. 
  

  

  road, 
  stopping 
  at 
  convenient 
  centers 
  and 
  driving 
  to 
  the 
  lake 
  or 
  river. 
  

   During 
  this 
  trip 
  45 
  lakes 
  and 
  15 
  rivers 
  were 
  examined, 
  with 
  the 
  re- 
  

   sults 
  as 
  herein 
  given. 
  

  

  Samples 
  of 
  the 
  shells 
  obtained 
  by 
  the 
  authors 
  at 
  the 
  different 
  lo- 
  

   caHties 
  herein 
  mentioned 
  have 
  been 
  identified 
  by 
  Mr. 
  H. 
  Walton 
  

   Clark, 
  of 
  the 
  biological 
  station 
  at 
  Fairport, 
  Iowa. 
  Similar 
  samples 
  

   have 
  been 
  tested 
  as 
  to 
  their 
  mercantile 
  value 
  by 
  Mr. 
  John 
  Southall, 
  

   of 
  the 
  same 
  station. 
  

  

  In 
  order 
  to 
  understand 
  the 
  geographic 
  distribution 
  of 
  the 
  mussels, 
  

  

  it 
  is 
  necessary 
  to 
  review 
  so 
  much 
  of 
  the 
  glacial 
  geology 
  as 
  will 
  explain 
  

  

  the 
  present 
  location 
  of 
  the 
  lakes 
  and 
  rivers 
  as 
  well 
  as 
  their 
  former 
  

  

  relations. 
  

  

  GLACIAL 
  ORIGIN 
  OF 
  LAKES 
  AND 
  RIVERS. 
  

  

  Minnesota 
  was 
  formerly 
  covered 
  by 
  a 
  great 
  glacier 
  or 
  ice 
  sheet, 
  

   which 
  came 
  down 
  from 
  the 
  northeast 
  and 
  flowed 
  across 
  the 
  State 
  

   into 
  Iowa. 
  When 
  the 
  glacier 
  melted, 
  the 
  gravel, 
  sand, 
  and 
  clay 
  

   which 
  it 
  carried 
  was 
  spread 
  out 
  over 
  the 
  surface 
  of 
  the 
  underlying 
  

   rocks 
  and 
  has 
  remained 
  there 
  ever 
  since. 
  The 
  only 
  exception 
  is 
  a 
  

   narrow 
  strip 
  bordering 
  the 
  Mississippi 
  River 
  from 
  Lake 
  Pepin 
  south- 
  

   ward, 
  and 
  thus 
  outside 
  of 
  the 
  present 
  survey. 
  North 
  of 
  Lake 
  Superior 
  

   tliis 
  drift, 
  as 
  it 
  is 
  called, 
  is 
  thin 
  in 
  places, 
  but 
  elsewhere 
  it 
  averages 
  

   from 
  100 
  to 
  150 
  feet 
  in 
  thickness 
  and 
  effectually 
  conceals 
  the 
  bedrock. 
  

  

  Both 
  glacier 
  and 
  drift 
  have 
  made 
  the 
  contour 
  of 
  the 
  State 
  more 
  

   level 
  and 
  uniform 
  than 
  it 
  was 
  before. 
  The 
  glacier 
  accomplished 
  this 
  

   by 
  grinding 
  off 
  the 
  ridges 
  and 
  elevations, 
  while 
  the 
  drift 
  assisted 
  by 
  

   filhng 
  in 
  such 
  hollows 
  as 
  were 
  left 
  by 
  the 
  glacier. 
  In 
  some 
  places 
  

   the 
  material 
  of 
  the 
  drift 
  has 
  been 
  subsequently 
  worked 
  over 
  and 
  

   arranged 
  in 
  layers 
  by 
  the 
  streams 
  and 
  rivers 
  that 
  flowed 
  from 
  beneath 
  

   the 
  glacier, 
  but 
  in 
  most 
  places 
  it 
  is 
  still 
  unstratified. 
  During 
  the 
  

   melting 
  of 
  tliis 
  great 
  glacier 
  there 
  were 
  successive 
  periods 
  of 
  advanc- 
  

   ing, 
  halting, 
  and 
  retreating, 
  and 
  at 
  such 
  times 
  the 
  drift 
  accumulated 
  

   in 
  long 
  hills 
  or 
  ridges 
  called 
  moraines, 
  parallel 
  with 
  the 
  edge 
  of 
  the 
  

   glacier. 
  Twelve 
  of 
  these 
  moraines 
  may 
  be 
  traced 
  across 
  the 
  State 
  

   in 
  various 
  directions, 
  and 
  while 
  they 
  are 
  only 
  from 
  25 
  to 
  75 
  feet 
  in 
  

   height, 
  they 
  are 
  still 
  sufficient 
  to 
  determine 
  the 
  general 
  drainage 
  

   of 
  the 
  State. 
  They 
  have 
  the 
  further 
  effect 
  of 
  rendering 
  any 
  free 
  

   drainage 
  impossible, 
  and 
  thus 
  they 
  give 
  rise 
  to 
  the 
  remarkable 
  number 
  

   of 
  lakes 
  that 
  are 
  found 
  in 
  Douglas, 
  Ottertail, 
  Itasca, 
  and 
  other 
  coun- 
  

   ties. 
  Furthermore, 
  the 
  streams 
  and 
  rivers 
  are 
  very 
  winding, 
  and 
  have 
  

   a 
  uniform 
  current, 
  and 
  there 
  are 
  plenty 
  of 
  sand 
  and 
  gravel 
  beds 
  in 
  

   the 
  bottom 
  of 
  the 
  lakes 
  and 
  rivers, 
  and 
  not 
  very 
  much 
  mud. 
  AU 
  

   these 
  conditions 
  are 
  favorable 
  for 
  mussel 
  growth 
  and 
  propagation 
  

   and 
  profoundly 
  influence 
  the 
  mussel 
  fauna. 
  

  

  When 
  the 
  melting 
  ice 
  sheet 
  had 
  receded 
  beyond 
  the 
  moraine 
  or 
  

   watershed 
  which 
  separated 
  the 
  basin 
  of 
  the 
  Minnesota 
  River 
  from 
  

  

  