CLAYS  OF  CAPE  COD,  MASSACHUSETTS.  435 
clays  of  the  Jacob  formation  vary  from  brown  and  greenish  in  the  more  clayey  varieties 
to  gray  and  yellowish  in  the  sandy  types. 
Character  of  the  clays  of  the  MontauJc  drift. — Under  this  head  is  included  the  clayey  and 
nearly  pebble-free  phases  of  the  Montauk  drift,  which  owe  their  origin  to  the  reworking 
practically  in  situ  of  the  Gardiner  clay  by  the  Montauk  ice  sheet.  It  differs  from  the 
unaltered  clays  in  the  absence  of  the  minute  but  highly  perfect  and  regular  lamination 
characteristic  of  the  latter,  exhibiting  rather  an  irregular  and  somewhat  indefinite  banding, 
such  as  might  be  expected  to  result  from  accumulation  beneath  an  ice  sheet.  The  most 
conspicuous  point  of  difference,  however,  is  the  presence  of  granitic  pebbles,  either  isolated 
or  in  layers  or  pockets,  or  of  an  occasional  erratic  bowlder,  indicating  a  glacial  origin  in 
contradistinction  to  the  interglacial  origin  of  the  Gardiner  clay. 
DESCRIPTIONS  OF  THE  CLAYS  BY  LOCALITIES. 
Promncetown  region. — West  of  the  bluffs  terminating  the  highland  northwest  of  North 
Truro  the  cape  is  made  up  of  a  series  of  curving  wave-built  spits  which  have  become  con- 
nected into  a  solid  mass  now  covered  with  dunes  rising  to  heights  of  from  80  to  100  feet 
above  sea  level.  Except  for  the  vegetable  muck  about  the  marshes  and  thin  beds  of  mucky 
silts  in  the  ponds  between  the  sand  hills,  nothing  in  the  shape  of  clay  is  found  near  the  sur- 
face in  the  Provincetown  region.  The  Gardiner  clay,  as  well  as  the  underlying  Cretaceous 
beds,  wThich  doubtless  include  more  or  less  clay,  probably  once  extended  over  this  region, 
but  were  subsequently  eroded  by  the  agency  of  waves  and  shore  currents  to  a  depth,  accord- 
ing to  a  recent  well  boring,  of  at  least  100  feet  below  the  present  sea  level,  before  the  waves 
and  winds  built  up  the  bars,  beaches,  and  dunes  constituting  the  present  surface.  At  a 
depth  of  140  feet  a  gray  clay  was  encountered  which,  though  probably  Cretaceous,  some- 
what resembles  the  Gardiner.  It  is  said  to  be  of  considerable  thickness,  but  no  accurate 
figures  were  obtainable. 
Truro  region. — This  term  as  here  used  includes  the  highlands  extending  northward  from 
Pamet  River  to  the  northwest  end  of  the  bluff  near  Moon  Pond.  The  best  exposures  of 
clay  are  found  in  the  bluff  near  Highland  Light,  but  impure  clays  or  clayey  sands  occur  at 
a  considerable  number  of  other  points. 
The  clay  near  Highland  Light,  which  is  of  the  typical  Gardiner  type,  first  appears  at  the 
top  of  the  bluff  at  a  point  about  one-half  mile  north  of  the  light-house.  It  continues  along 
the  top  for  some  distance,  then  gradually  sinks  as  the  gentle  southward  dip  brings  it  nearer 
the  beach.     At  Highland  Light  the  section  shown  by  the  bluff  is  as  follows: 
Section  at  Highland  Light,  Cape  Cod. 
Feet. 
Dune  sand 5 
Yellowish  quartz  sand,  with  erratic  pebbles  near  the  surface 35 
Tough  gray  to  nearly  black  Gardiner  clay,  interstratified  with  sand  near  top..  45 
Iron-stained  gravel,  sometimes  partly  cemented 15 
White  sand 15 
Beyond  the  light-house  the  clay  continues  with  the  same  southward  dip,  outcropping 
with  slight  undulations  for  about  half  a  mile,  beyond  which  it  is  not  seen  because  of  the 
covering  of  sandy  talus  for  a  distance  of  a  mile.  At  this  point  about  20  feet  of  clay  are 
exposed,  the  top  being  40  feet  above  the  beach.  It  is  not  seen  again  to  the  south,  but  as 
the  upper  part  of  the  bluffs  is  entirely  of  sand  it  is  evident  that  the  clay  sinks  below  sea  level 
in  this  direction.  The  clay  also  comes  close  to  the  surface  on  the  flats  near  the  highway 
west  of  the  light-house,  where  it  is  reported  to  be  found  over  a  considerable  area.  Although 
the  clay  occurs  in  vast  quantities  and  is  easily  accessible  from  the  land  side,  no  material  use 
of  it  appears  to  have  been  made.  Similar  clays,  however,  were  elsewhere  extensively  used 
in  the  construction  of  the  State  roads,  and  the  clays  near  Highland  Light  might  be  used  to 
advantage  for  similar  purposes.  Owing  to  exposure  to  the  open  sea  it  will  probably  never 
be  practicable  to  work  the  clays  from  the  ocean  side.  It  is  not  impossible,  however,  that 
at  some  future  time,  when  more  accessible  deposits  elsewhere  are  exhausted,  the  clays  under 
