﻿T. 
  M'KENNY 
  HUGHES 
  OST 
  THE 
  GEOLOGY 
  OE 
  ANGLESEY. 
  

  

  21 
  

  

  The 
  beds 
  from 
  Pwllgoch 
  to 
  Caemawr 
  are 
  probably 
  Arenig 
  (lower- 
  

   most 
  Bala 
  of 
  some). 
  At 
  Pwllgoch 
  we 
  have 
  the 
  ironstone 
  so 
  charac- 
  

   teristic 
  of 
  Arenig, 
  and 
  at 
  Caemawr 
  we 
  find 
  Arenig 
  Graptolites 
  (see 
  

   p. 
  26). 
  

  

  Where 
  the 
  dykes 
  cross 
  the 
  series 
  at 
  Llyseinion 
  (fig. 
  3), 
  sandy 
  beds 
  

   begin 
  to 
  appear, 
  and 
  near 
  Yr 
  efail 
  newydd 
  predominate 
  over 
  the 
  

   shales, 
  as 
  seen 
  in 
  several 
  large 
  quarries 
  near 
  the 
  road. 
  The 
  low 
  

   ground 
  S.W. 
  of 
  this, 
  by 
  Plas 
  Llandyfrydog, 
  may 
  be 
  partly 
  due 
  to 
  

   faulted 
  broken 
  rock 
  ; 
  for 
  it 
  is 
  probable 
  that 
  for 
  every 
  fault 
  I 
  have 
  

   detected 
  in 
  this 
  turf-covered 
  ground 
  I 
  have 
  missed 
  10 
  or 
  may 
  be 
  100. 
  

  

  The 
  lowest 
  sandstones 
  seen 
  pass 
  down 
  into 
  the 
  conglomerates 
  of 
  

   y 
  Eoel, 
  E.N.E. 
  of 
  Llanerchymedd 
  ; 
  and 
  south 
  of 
  y 
  Eoel, 
  apparently 
  

   cropping 
  out 
  from 
  below 
  the 
  conglomerate, 
  there 
  are 
  green 
  chloritic 
  

   schistose 
  rocks, 
  weathering 
  olive 
  and 
  brown, 
  which 
  are 
  probably 
  

   part 
  of 
  the 
  Bangor 
  beds 
  (Pebidian). 
  There 
  is 
  no 
  rock 
  exposed 
  in 
  

   the 
  valley 
  immediately 
  south 
  of 
  this, 
  where 
  the 
  word 
  Llanerchymedd 
  

   is 
  engraved 
  on 
  the 
  survey 
  map. 
  There 
  is 
  probably 
  a 
  fault 
  through 
  

   here, 
  with 
  a 
  downthrow 
  on 
  the 
  south 
  repeating 
  the 
  Cambrian 
  con- 
  

   glomerates, 
  which 
  can 
  be 
  traced 
  from 
  nearly 
  west 
  of 
  Pen 
  cefn, 
  through 
  

   the 
  cottage 
  near 
  the 
  junction 
  of 
  the 
  roads 
  from 
  Plas 
  Coedana 
  and 
  

   Capel 
  Penial, 
  and 
  sweeping 
  round 
  north 
  of 
  the 
  windmill, 
  and 
  crossing 
  

   the 
  road 
  to 
  Llanfihangeltre'rbeirdd 
  obliquely, 
  are 
  seen, 
  east 
  of 
  the 
  

   farm 
  road 
  leading 
  to 
  Tyddynbach, 
  resting 
  with 
  a 
  sharp 
  line 
  of 
  junc- 
  

   tion 
  upon 
  schists 
  similar 
  to 
  those 
  mentioned 
  above 
  as 
  cropping 
  out 
  

   from 
  below 
  the 
  conglomerate 
  on 
  y 
  Eoel 
  (see 
  fig. 
  3). 
  Near 
  Pen 
  cefn 
  

   the 
  conglomerates 
  pass 
  up 
  into 
  fossiliferous 
  flaggy 
  grits 
  and 
  sand- 
  

   stones 
  (see 
  p. 
  25) 
  . 
  

  

  Pig. 
  4. 
  — 
  Section 
  from 
  Mynyddmawr 
  to 
  Llanerchymedd 
  and 
  Pen 
  cefn. 
  

  

  Llanerchymedd. 
  

  

  N.E 
  

   Pen 
  cefn. 
  

  

  a. 
  Sandstone 
  and 
  subordinate 
  shale. 
  

  

  b. 
  Ashy 
  -looking 
  brecciated 
  con- 
  

  

  glomerate. 
  

  

  c. 
  Sandstone 
  with 
  subordinate 
  shale. 
  

  

  d. 
  Sandstone. 
  

  

  e. 
  Grit 
  with 
  annelid-tubes. 
  

  

  /. 
  Quartz-jasper 
  conglomerate. 
  

   g. 
  Archaean. 
  

  

  If 
  we 
  now 
  make 
  a 
  traverse 
  from 
  near 
  Pen 
  cefn 
  (mentioned 
  above) 
  

   S.W., 
  about 
  1 
  mile 
  to 
  Mynyddmawr 
  (see 
  fig. 
  4), 
  we 
  leave 
  the 
  con- 
  

   glomerate 
  and 
  grit 
  dipping 
  westerly, 
  and 
  crossing 
  the 
  valley 
  that 
  

   runs 
  south 
  from 
  Llanerchymedd, 
  find 
  the 
  brown 
  sandstones 
  in 
  a 
  

   quarry 
  near 
  the 
  n 
  of 
  Cerygydrydion 
  dipping 
  at 
  about 
  75°, 
  still 
  

   westerly. 
  Near 
  Cilgwyn, 
  however, 
  we 
  find 
  ourselves 
  on 
  the 
  other 
  

   side 
  of 
  what 
  is 
  probably 
  a 
  broken 
  synclinal, 
  and 
  in 
  a 
  quarry 
  near 
  

  

  