﻿22 
  H. 
  E. 
  Gregory 
  — 
  Gravels 
  at 
  Cuzeo. 
  

  

  along 
  the 
  Huatanay 
  and 
  on 
  the 
  Anta 
  divide. 
  The 
  contributary 
  

   area 
  on 
  the 
  west 
  and 
  northwest 
  is 
  underlaid 
  by 
  brown 
  and 
  grey 
  

   sandstone, 
  chiefly 
  the 
  former. 
  Sandstone 
  furnished 
  the 
  bulk 
  

   of 
  the 
  material, 
  and 
  in 
  the 
  Ayahuaycco 
  Quebrada 
  fully 
  95 
  per 
  

   cent 
  of 
  the 
  pebbles 
  have 
  this 
  origin, 
  igneous 
  pebbles 
  being 
  

   nearly 
  absent. 
  Along 
  the 
  Huatanay 
  tributaries 
  the 
  sandstone 
  

   is 
  still 
  dominant, 
  but 
  limestone 
  makes 
  up 
  about 
  10 
  per 
  cent 
  

   and 
  diabase 
  3 
  per 
  cent 
  to 
  5 
  per 
  cent. 
  Bowlders 
  of 
  limestone 
  

   exceeding 
  5 
  feet 
  in 
  diameter 
  are 
  found 
  in 
  this 
  area. 
  The 
  rel- 
  

   ative 
  scarcity 
  of 
  large 
  bowlders 
  of 
  sandstone 
  appears 
  to 
  be 
  due 
  

   to 
  the 
  fact 
  that 
  closely 
  spaced 
  intersecting 
  joint-planes 
  break 
  

   up 
  this 
  rock 
  into 
  cubes 
  of 
  a 
  few 
  inches 
  on 
  a 
  side. 
  That 
  brown 
  

   sandstone 
  should 
  rank 
  first 
  as 
  the 
  source 
  of 
  material 
  is 
  due 
  (1) 
  

   to 
  the 
  more 
  precipitous 
  slopes 
  on 
  the 
  west 
  border 
  of 
  the 
  fan 
  ; 
  

   (2) 
  to 
  the 
  more 
  abundant 
  fragmental 
  waste 
  resulting 
  from 
  the 
  

   weathering 
  of 
  sandstone 
  as 
  compared 
  with 
  limestone 
  ; 
  (3) 
  to 
  the 
  

   fact 
  that 
  those 
  portions 
  of 
  the 
  fan 
  where 
  limestone 
  and 
  diabase 
  

   bowlders 
  were 
  formerly 
  most 
  abundant 
  (e.g., 
  along 
  the 
  Hua- 
  

   tanay 
  river) 
  have 
  been 
  most 
  completely 
  removed 
  by 
  stream 
  

   erosion. 
  It 
  was 
  noticed 
  that 
  limestone 
  fragments 
  are 
  more 
  

   abundant 
  in 
  the 
  topmost 
  beds 
  of 
  the 
  fan 
  — 
  a 
  natural 
  consequence 
  

   of 
  the 
  fact 
  that 
  the 
  limestone 
  occupied 
  southern 
  slopes 
  and 
  was 
  

   more 
  or 
  less 
  protected 
  by 
  a 
  cover 
  of 
  snow 
  and 
  ice 
  during 
  the 
  

   early 
  part 
  of 
  the 
  period 
  of 
  excessive 
  aggradation. 
  So 
  far 
  as 
  

   observed, 
  coarse, 
  sandstone 
  gravel 
  forms 
  the 
  layer 
  immediately 
  

   in 
  contact 
  with 
  bed 
  rock. 
  

  

  Three 
  types 
  of 
  material 
  are 
  represented 
  in 
  the 
  fan 
  : 
  gravel, 
  

   coarse 
  to 
  very 
  coarse 
  ; 
  sand, 
  fine, 
  compact 
  ; 
  and 
  adobe. 
  The 
  

   structure 
  and 
  composition 
  of 
  the 
  gravel 
  and 
  sand 
  have 
  been 
  dis- 
  

   cussed. 
  Brown 
  adobe 
  occupies 
  about 
  an 
  acre 
  on 
  the 
  west 
  bank 
  

   of 
  the 
  Ayahuaycco 
  Quebrada, 
  where 
  it 
  attains 
  a 
  thickness 
  of 
  

   thirty 
  feet 
  and 
  includes 
  three 
  bands 
  of 
  white 
  lime 
  silt. 
  The 
  

   material 
  lies 
  in 
  horizontal 
  beds, 
  is 
  impalpably 
  fine 
  but 
  firm 
  and 
  

   compact. 
  The 
  adobe 
  and 
  associated 
  beds 
  are 
  alike 
  highly 
  cal- 
  

   careous 
  and 
  are 
  traversed 
  by 
  vertical 
  root 
  tubes 
  encrusted 
  with 
  

   lime. 
  This 
  deposit, 
  as 
  well 
  as 
  the 
  less 
  extensive 
  accumulations 
  

   near 
  Santa 
  Ana 
  church 
  and 
  at 
  the 
  mouth 
  of 
  the 
  Ayahuaycco, 
  

   differ 
  in 
  no 
  essential 
  from 
  the 
  beds 
  of 
  adobe 
  and 
  unconsolidated 
  

   limestone 
  abundantly 
  displayed 
  in 
  the 
  Cuzco 
  Valley 
  beyond 
  the 
  

   border 
  of 
  the 
  fan. 
  They 
  are 
  believed 
  to 
  represent 
  the 
  shores 
  

   of 
  ephemeral 
  lakes 
  which 
  existed 
  contemporaneously 
  with 
  the 
  

   gravel-bearing 
  streams. 
  

  

  Age 
  of 
  Deposits. 
  

  

  As 
  previously 
  stated, 
  the 
  Cuzco 
  gravels 
  are 
  believed 
  to 
  have 
  

   reached 
  their 
  greatest 
  extent 
  and 
  thickness 
  in 
  late 
  Pleistocene 
  

   times. 
  More 
  recent 
  deposits 
  have, 
  however, 
  been 
  superposed. 
  

  

  