Vol.  49.] 
PEBBLES  FROM  THE  GLACIAL  DRIFT. 
315 
these,  and  it  proves  to  be  a  chert  with  indistinct  signs  of  organic 
remains,  some  of  which  Dr.  G.  J.  Hinde  pronounces  to  be  sponge- 
spicules.  Mr.  White  also  found  a  pebble  of  grey  chert  with  casts 
of  detached  joints  of  crinoids  at  Bowsey  Hill,  and  another  of  a 
similar  character  at  Ashley  Hill.  Both  the  black  and  grey  cherts 
may  well  be  derived  from  the  Carboniferous  Series  of  the  north. 
Besides  these  cherts,  Mr.  White  found  at  Ashley  Hill  occasional 
red  quartzite-pebbles,  unlike  any  which  I  have  found  in  undoubted 
Westleton  Shingle,  and  they  may  very  probably  be  derived  from  the 
Glacial  Gravel.  I  should  therefore  draw  the  line  of  the  southern 
boundary  for  Glacial  Drift  materials  from  Buscomb,  south  of  Bowsey 
Hill,  Ashley  Hill,  and  Cookham  Dean,  to  the  highest  terrace  of 
Thames  Gravel  near  Maidenhead.1 
TII.  Maidenhead. 
South  of  Ashley  Hill,  Bowsey  Hill,  and  the  Cookham  hills,  there 
is  a  stretch  of  low-lying  country  reaching  from  the  Thames  at  War- 
grave  to  the  Thames  at  Maidenhead  (see  fig.  2,  p.  313),  and  it  has 
been  suggested  that  the  river  may  once  have  flowed  along  this  line, 
but  the  gravels  afford  no  evidence  in  favour  of  such  a  contention.2 
I  have  already  described  the  Sonning  Plateau,  which  is  capped  by 
Thames  Gravel  and  overlooks  this  low  ground  from  the  west.  On 
the  east  there  is  a  similar  plateau — -a  portion  of  the  highest  terrace 
of  Mr.  Whitaker’s  account.3  This  plateau,  owing  to  denudation 
since  its  deposition,  has  come  to  project  into  the  valley  south  of 
Maidenhead,  and  overlooks  on  one  side  the  Thames,  and  on  the 
other  the  low-lying  ground  towards  Heading.  Its  level  is  about 
150  feet  O.D.,  and  a  good  section  is  usually  open  in  a  gravel-pit  at 
Shoppenhanger’s  Harm.  The  gravel  consists  of  : — 
(a)  Local  Material. — Tertiary  flint-pebbles,  green-coated  flints,  black  flints, 
sarsen-stone. 
(b)  Southern  Drift  Material. — Brown  subangular  flints,  flint-pebbles,  Lower 
Greensand  fragments,  small  quartz. 
(c)  Glacial  Gravel  Material. — Pebbles  of  red  and  grey  quartzite,  and  large 
quartz -pebbles. 
In  a  gravel-pit  close  to  the  Great  Western  Hailway,  a  little  west 
of  Maidenhead.  Station,  on  the  same  plateau,  Mr.  B.  S.  Herries  found 
a  block  of  quartz-conglomerate  measuring  about  6  x  2|  inches. 
This  terrace  is  some  100  feet  lower  than  the  gravel  already  noted  at 
Hillgrove  Farm,  above  Cookham,  and  is  the  highest  of  the  three 
terraces  at  Maidenhead.  We  have  therefore  a  series  of  gravels 
showing  the  gradual  excavation  of  this  part  of  the  Thames  Talley, 
and,  as  all  the  gravels  here  (so  far  as  I  have  seen)  contain  Northern 
1  Ashley  Hill  is  shown  in  outline  in  fig.  2,  p.  313,  as  it  lies  two  miles  west 
of  the  actual  line  of  section. 
2  See  Prestwick,  Quart.  Journ.  Geol.  Soc.  vol.  xlvi.  (1890)  p.  141,  note. 
3  ‘Geology  of  London,’  vol.  i.  p.  391,  &  fig.  70,  1889,  and 'Mem.  Geol. 
Surv.  (Sheet  vii.)  p.  82,  fig.  12,  1864. 
T  2 
