Yol.  49.]  LLANDOVERY  AND  ASSOCIATED  ROCKS  OF  CORWEN.  429 
The  Berwyn  Hills,  as  is  well  known,  form  a  tolerably  well- 
defined  dome,  which  is  made  up  almost  entirely  of  Bala  rocks.  But 
the  dome  is  by  no  means  perfect,  and  here,  at  its  northern  margin, 
we  have  found  that  it  is  cut  through  by  a  series  of  parallel  faults 
which  run  nearly  due  east-and-west,  and,  throwing  towards  the 
north,  have  apparently  determined  the  course  of  the  Dee,  for  the 
direction  of  the  valley  is  parallel  to  the  direction  of  these  faults. 
The  southern  bank  of  the  valley  close  to  Corwen  is  practically 
the  denuded  face  of  one  of  these  faults  and  forms  a  scarp,  from  the 
top  of  which  the  dome  proper  slopes  gently  upward  towards  the 
south  with  an  undulating  or  hilly  surface.  The  top  of  the  scarp 
is  some  900  feet  above  the  sea.  Farther  east,  towards  Pen-y-glog, 
the  slope  of  the  southern  bank  of  the  Dee  is  much  gentler  and  is 
interrupted  by  several  low  hills. 
It  is  the  Corwen  Grit  which  enables  us  to  make  out  the  geology 
of  the  district ;  and  accordingly  our  description  of  the  structure 
will  begin  with  an  account  of  the  lie  of  this  grit.  It  should  be 
premised  that  there  is  near  Corwen  an  important  fault,  which  runs 
28°  W.  of  1ST.,  and  for  a  considerable  distance  lies  in  the  course  of 
Nant  Cawrddu,  and  that  a  nearly  parallel  fault  occurs  in  Nant 
Llechog,  near  Pen-y-glog.  For  the  sake  of  clearness,  the  area  between 
these  two  faults  will  be  described  first. 
1.  Main  Outcrop  of  Grit. — Ascending  the  hill  from  Corwen  to  the 
Prince-of- Wales  Memorial  on  Pen-y-pigin,  about  half-way  up  we 
pass  a  quarry  in  grit,  which  is  worked  for  road-metal.  The  grit 
here,  however,  forms  only  a  small  patch,  and,  as  will  be  shown 
farther  on,  has  been  let  down  by  a  fault.  Continuing  along  the 
path,  we  find  the  main  outcrop  of  grit  just  outside  the  moorland 
boundary-wall  some  distance  higher  up.  The  grit  here  strikes  a 
little  north  of  east  to  the  point  known  as  Pen-y-pigin,  and  thence 
onward  it  forms  the  crest  of  the  cliff  nearly  as  far  as  the  stream 
which  runs  down  past  the  Rectory  (about  a  mile  east  of  Corwen). 
Here  it  is  covered  by  glacial  drift  for  a  short  distance,  but  is  found 
again  on  the  eastern  bank  of  the  stream  near  the  margin  of  the  hill. 
It  no  longer  forms  a  well-marked  cliff,  but  may  be  traced  along  the 
top  of  the  hill  till  it  sinks  into  a  tract  of  marshy  land  some  600  yards 
farther  east.  On  the  east  of  this  tract  it  reappears  and  may  be  fol¬ 
lowed  to  Nant  Llechog,  where  it  is  cut  off  by  the  Kant  Llechog  faults. 
Throughout  the  whole  of  this  distance  the  grit  dips  steeply  in  a  south¬ 
erly  direction,  i.  e.  into  the  hill,  and  in  places  is  nearly  vertical. 
But  in  Nant  Llechog  this  dip  is  continued  for  a  short  distance  only  ; 
a  little  farther  south  the  grit  rises  again,  dipping  at  alow  angle  towards 
the  north,  and  for  a  considerable  distance  forms  the  bed  of  the  stream. 
The  Bala  Beds  below  the  grit  show  that  the  synclinal  so  formed  is 
followed  on  the  north  by  a  faulted  anticlinal  (fig.  1,  p.  430) 
2.  Country  South  of  the  Main  Grit-band. — Near  Nant  Llechog,  as 
has  just  been  described,  the  grit  turns  up  towards  the  south  and 
forms  a  synclinal,  and  this  appears  to  be  the  case  throughout  the 
area.  Near  Corwen,  as  already  noticed,  the  main  band  of  grit  dips 
