In  Part  I  we  have  already  studied  the  agencies  of  denudation,
but  there  we  concerned  ourselves  principally  with  their  modes  of
operation  and  their  efficiency  in  destroying  old  rocks  and  in  fur-
nishing material  for  the  construction  of  new.  We  have  now  to
consider  these  agencies  from  a  somewhat  different  point  of  view;
to  determine  the  characteristic  forms  of  land  sculpture  which  they
produce  at  the  various  stages  of  their  work.
The  Sea.  — The  work  of  the  sea  is  confined  to  the  coast-line,
which  it  cuts  back  by  the  impact  of  its  waves  and  currents.  Speak-
ing broadly,  the  waves  do  but  little  effective  work  below  the  limits
of  low  tide,  and  advance  by  undermining  and  cutting  down  the
cliffs  which  form  the  coast.  The  result  of  the  work  is  to  form
a  platform  covered  by  shallow  water,  which  is  called  a  plain  of
marine  denudation.  As  observed  in  actual  cases,  these  platforms
are  narrow;  for  so  long  as  the  sea-level  remains  constant  with
reference  to  the  land,  there  is  a  limit  to  the  effective  assault  of
the  waves  upon  the  shore.  The  water  covering  the  platform  is
very  shallow,  and  only  in  exceptional  cases  do  the  waves  have
sufficient  power  to  overcome  the  friction  of  a  wide  platform.  The
materials  removed  from  the  land  are  piled  up  at  the  seaward  foot
of  the  platform  and  extend  it  in  that  direction.
