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MR.  H.  W.  MONCKTON  ON  BOULDERS  AND  [Aug.  1893, 
26.  On  the  Occurrence  of  Boulders  and  Pebbles  from  the  Glacial 
Drift  in  Gravels  south  of  the  Thames.  By  Horace  W. 
Monckton,  Esq.,  E.L.S.,  E.G.S.  (Bead  March  8th,  1893.) 
I.  Introduction. 
In  Hertfordshire,  Middlesex,  and  Buckinghamshire  the  Glacial 
Drift  consists  of  Boulder  Clay  and  also  of  extensive  sheets  of  gravel. 
The  reasons  for  classing  this  gravel  as  Glacial  Drift  are  given  by 
Mr.  Whitaker  in  his  ‘  Geology  of  London,’  1889,  vol.  i.  pp.  299- 
3(J0.  In  this  gravel  we  frequently  find  numerous  pebbles  or  boulders 
of  red  quartzites  and  sandstones,  black  cherty  pebbles,  blocks  of 
white  quartz  ;  and  occasionally  we  meet  with  boulders  of  igneous 
rock.  Now  all  these  materials  are  characteristic  of  the  Glacial 
Gravel  or  Northern  Drift,  but  are  practically  absent  from  the 
Southern  Drift,  as  has  been  pointed  out  by  Prof.  Prestwich.  Not 
only  are  materials  of  this  sort  wanting  or  very  rare  in  the  Southern 
Drift,  but  also  in  the  valley-gravels  of  what  I  may  call  the  4  Southern 
Drift  country,’  in  which  I  include  large  parts  of  Kent,  Surrey,  North 
Hampshire,  and  Berkshire.  It  is  true  that  very  rarely  boulders  of 
white  quartz  occur  in  these  gravels,  and  pebbles  of  various  old 
rocks  may  occasionally  be  found ;  these  last  are,  however,  quite 
unlike  the  red  quartzite  and  sandstone-pebbles  so  common  in  the 
Glacial  Drift,  and  probably  are  derived  from  Tertiary  pebble-beds. 
The  southern  limit  of  distribution  of  the  Northern  Drift  materials 
in  hill-  and  valley-gravels  very  nearly  coincides  with  the  Biver 
Thames,  at  least  in  the  country  east  of  Pangbourne,  but  not  abso¬ 
lutely  so,  and  the  object  of  the  present  paper  is  to  show  where  and 
to  what  extent  the  Glacial  Gravels  or  their  debris  are  now  to 
be  found  south  of  that  river.1  In  my  search  for  sections  and  in  the 
collection  of  facts  I  have  been  greatly  assisted  by  several  gentlemen, 
and  I  am  more  particularly  indebted  to  Messrs.  J.  H.  Blake  (of 
the  Geological  Survey)  and  B.  S.  Herries.  Prof.  Bonney  has  also 
very  kindly  assisted  me  in  the  examination  and  description  of  my 
specimens  and  microscope-slides. 
II.  Tilehurst. 
I  have  already  pointed  out 2  that  the  Glacial  Gravel  crosses  the 
Thames  above  Beading,  and  forms  the  plateau  above  which  the 
1  [I  am  unable  to  agree  with  the  opinion  expressed  in  the  discussion  on 
this  paper  that  the  narrow  valley  of  the  Thames  is  of  later  origin  than  the 
broad  valley  of  the  Kennet  (see  fig.  1,  p.  311).  I  believe  that  the  difference  in 
width  and  shape  of  the  valleys  is  due  to  the  nature  of  the  strata  in  which  they 
are  cut  out,  just  as  is  the  case  with  the  cuttings  on  the  Great  Western  Railway : 
^qo  ^onnlnS  cutting  being  wide,  the  Pangbourne  cutting  narrow. — May  18th, 
1893.] 
2  Quart.  Journ.  Geol.  Soc.  vol.  xlviii.  (1892)  p.  40. 
