﻿GEOLOGY. 
  63 
  

  

  ORIGIN. 
  

  

  Most 
  of 
  the 
  terrace 
  deposits 
  are 
  probably 
  of 
  marine 
  origin. 
  This 
  

   is 
  proved 
  in 
  the 
  case 
  of 
  the 
  most 
  typical 
  deposits 
  by 
  the 
  presence 
  of 
  

   the 
  pholas 
  borings 
  already 
  mentioned. 
  The 
  deposition 
  was 
  carried 
  

   on 
  in 
  shallow 
  water 
  and 
  much 
  of 
  the 
  material 
  was 
  derived 
  on 
  the 
  

   spot 
  from 
  the 
  wearing 
  away 
  of 
  the 
  shore 
  line 
  of 
  Monterey 
  shale, 
  the 
  

   fragments 
  of 
  which 
  were 
  not 
  always 
  subjected 
  to 
  much 
  polishing 
  

   before 
  being 
  deposited 
  and 
  protected 
  from 
  agencies 
  of 
  erosion. 
  

   These 
  deposits 
  give 
  undeniable 
  evidence 
  of 
  a 
  great 
  uplift 
  of 
  the 
  

   coast 
  during 
  Pleistocene 
  time. 
  It 
  seems 
  most 
  probable 
  that 
  the 
  

   terraced 
  surfaces 
  resulted 
  from 
  marine 
  planation 
  along 
  gradually 
  

   rising 
  shore 
  lines 
  and 
  that 
  the 
  formation 
  covering 
  them 
  represents 
  

   the 
  beach 
  and 
  shore 
  deposits. 
  The 
  rise 
  of 
  the 
  land 
  was 
  probably 
  too 
  

   rapid 
  and 
  the 
  amount 
  of 
  sediment 
  too 
  small 
  to 
  allow 
  much 
  off-shore 
  

   extension 
  of 
  the 
  deposition. 
  The 
  material 
  that 
  may 
  have 
  been 
  

   deposited 
  in 
  the 
  deeper 
  places 
  determined 
  by 
  the 
  depressions 
  in 
  the 
  

   topography 
  has 
  since 
  probably 
  been 
  largely 
  removed 
  by 
  erosion. 
  

   The 
  terrace 
  deposits 
  themselves 
  have 
  been 
  extensively 
  eroded 
  and 
  

   in 
  many 
  places 
  are 
  left 
  as 
  mere 
  remnants. 
  Some 
  of 
  them 
  have 
  no 
  

   doubt 
  been 
  subsequently 
  added 
  to 
  by 
  wind-blown 
  sand. 
  

  

  It 
  is 
  probable 
  that 
  some 
  of 
  the 
  terraces 
  and 
  horizontal 
  Pleistocene 
  

   deposits 
  along 
  valleys 
  have 
  been 
  formed 
  by 
  streams. 
  Most 
  of 
  the 
  

   valley 
  fillings 
  were 
  probably 
  laid 
  down 
  in 
  this 
  way. 
  At 
  the 
  mouths 
  

   of 
  some 
  canyons, 
  as 
  along 
  the 
  western 
  side 
  of 
  Graciosa 
  Canyon, 
  

   Pleistocene 
  deposits 
  have 
  been 
  built 
  up 
  in 
  the 
  shape 
  of 
  detrital 
  fans, 
  

   which 
  have 
  since 
  been 
  carved 
  into 
  flat-topped, 
  steep-sided 
  blocks 
  by 
  

   recent 
  streams. 
  

  

  DTJNE 
  SAND. 
  

  

  The 
  prevailing 
  northwesterly 
  wind 
  from 
  the 
  ocean 
  has 
  amassed 
  

   great 
  deposits 
  of 
  sand 
  in 
  places 
  along 
  the 
  coast. 
  The 
  process 
  has 
  

   probably 
  been 
  going 
  on 
  all 
  through 
  the 
  Quaternary 
  period 
  and 
  it 
  is 
  

   hard 
  to 
  distinguish 
  the 
  older 
  of 
  the 
  eolian 
  deposits 
  from 
  those 
  partially 
  

   or 
  entirely 
  of 
  marine 
  deposition. 
  The 
  line 
  of 
  contact 
  of 
  these 
  forma- 
  

   tions 
  as 
  mapped 
  is 
  arbitrary. 
  

  

  The 
  greatest 
  mass 
  of 
  dune 
  sand 
  occurs 
  at 
  the 
  northwest 
  end 
  of 
  the 
  

   Casmalia 
  Hills, 
  where 
  the 
  gradual 
  slope 
  down 
  to 
  the 
  sea 
  from 
  an 
  eleva- 
  

   tion 
  of 
  about 
  1,200 
  feet 
  is 
  covered 
  by 
  loose, 
  yellow 
  sand 
  of 
  probable 
  

   eolian 
  origin. 
  This 
  drifts 
  about 
  incessantly 
  and 
  is 
  probably 
  still 
  in 
  

   the 
  process 
  of 
  collecting, 
  being 
  supplied 
  from 
  the 
  long, 
  low, 
  open 
  

   shore 
  to 
  the 
  north 
  and 
  held 
  in 
  check 
  by 
  the 
  bulwark 
  of 
  the 
  Casmalia 
  

   Hills 
  on 
  the 
  south. 
  This 
  deposit 
  has 
  a 
  thickness 
  of 
  several 
  hundred 
  

   feet. 
  At 
  its 
  base 
  along 
  the 
  coast 
  is 
  exposed 
  a 
  basal 
  layer 
  of 
  large 
  

   bowlders 
  and 
  horizontally 
  stratified 
  sand. 
  The 
  original 
  slope 
  of 
  the 
  

  

  