﻿28 
  H. 
  E. 
  Gregory 
  — 
  Gravels 
  at 
  Cuzco. 
  

  

  The 
  lower 
  Ayahuaycco 
  Quebrada 
  furnishes 
  evidence 
  of 
  the 
  

   manner 
  in 
  which 
  slides 
  modify 
  valley 
  form 
  and 
  aid 
  in 
  the 
  

   burial 
  of 
  extraneous 
  matter. 
  The 
  stream 
  has 
  occupied 
  this 
  

   portion 
  of 
  its 
  valley 
  since 
  about 
  1870, 
  when 
  an 
  artificial 
  read- 
  

   justment 
  of 
  drainage 
  was 
  effected. 
  During 
  these 
  forty 
  years 
  

   the 
  stream 
  has 
  cut 
  a 
  ravine 
  twenty 
  to 
  thirty 
  feet 
  deep 
  and 
  

   about 
  twenty 
  feet 
  wide. 
  Three 
  slides 
  are 
  visible 
  within 
  one 
  

   hundred 
  feet. 
  The 
  one 
  nearest 
  the 
  stream's 
  mouth 
  covers 
  

   about 
  twenty 
  square 
  feet 
  and 
  is 
  pasted 
  against 
  the 
  vertical 
  wall 
  

   as 
  a 
  patch 
  two 
  feet 
  thick. 
  In 
  this 
  case 
  stratification 
  is 
  well 
  

   marked, 
  but 
  the 
  break 
  is 
  so 
  completely 
  healed 
  that, 
  without 
  

   the 
  discordance 
  in 
  bedding, 
  the 
  displacement 
  could 
  not 
  be 
  

   detected. 
  The 
  second 
  slide 
  blocked 
  the 
  stream, 
  which, 
  rising 
  

   to 
  the 
  crest 
  of 
  the 
  dam, 
  cut 
  downward 
  and 
  formed 
  a 
  new 
  floor. 
  

   Later 
  trenching 
  developed 
  a 
  terrace 
  12x20 
  feet, 
  the 
  edges 
  of 
  

   which 
  reveal 
  material 
  identical 
  with 
  the 
  gravel 
  in 
  the 
  original 
  

   bank. 
  The 
  material 
  of 
  the 
  slide, 
  the 
  material 
  deposited 
  by 
  

   the 
  stream, 
  the 
  material 
  fallen 
  on 
  the 
  terrace, 
  and 
  the 
  material 
  

   of 
  the 
  standing 
  wall 
  are 
  indistinguishable 
  in 
  structure, 
  texture 
  

   and 
  composition. 
  The 
  fact 
  of 
  the 
  slide 
  is 
  demonstrated 
  by 
  a 
  

   broken 
  lense 
  of 
  sand 
  revealed 
  by 
  an 
  artificial 
  trench. 
  From 
  

   the 
  gravels 
  of 
  the 
  slide 
  midway 
  between 
  top 
  and 
  bottom 
  were 
  

   excavated 
  two 
  pieces 
  of 
  imported 
  pottery, 
  relics 
  carried 
  by 
  the 
  

   stream 
  or 
  fallen 
  from 
  the 
  top 
  of 
  the 
  bank. 
  A 
  deeper 
  cutting 
  

   of 
  the 
  channel, 
  accompanied 
  by 
  smoothing 
  of 
  the 
  slope, 
  would 
  

   have 
  left 
  those 
  fragments 
  of 
  household 
  furniture 
  firmly 
  

   embedded 
  in 
  a 
  wall 
  of 
  gravel 
  beneath 
  twenty 
  feet 
  of 
  sediment, 
  

   all 
  so 
  like 
  the 
  gravel 
  of 
  the 
  original 
  fan 
  as 
  not 
  to 
  be 
  differenti- 
  

   ated 
  with 
  certainty. 
  It 
  seems 
  reasonable 
  to 
  suppose 
  that 
  much 
  

   greater 
  thicknesses 
  of 
  sediment 
  forming 
  much 
  higher 
  banks 
  

   of 
  gravel 
  may 
  have 
  passed 
  through 
  similar 
  stages. 
  

  

  JErosional 
  History. 
  — 
  The 
  present 
  position 
  and 
  structure 
  of 
  

   the 
  Cuzco 
  gravels 
  and 
  the 
  general 
  physiographic 
  relations 
  of 
  

   the 
  area 
  suggest 
  the 
  outline 
  of 
  the 
  original 
  deposits 
  as 
  indi- 
  

   cated 
  on 
  the 
  map 
  (fig. 
  2). 
  The 
  dissection 
  of 
  the 
  fan 
  probably 
  

   began 
  with 
  the 
  establishment 
  of 
  permanent 
  drainage 
  in 
  the 
  

   Sappi, 
  whose 
  relatively 
  large 
  watershed 
  and 
  its 
  fall 
  of 
  about 
  

   400 
  feet 
  per 
  mile 
  gave 
  it 
  considerable 
  erosive 
  power 
  over 
  the 
  

   gravels 
  marking 
  its 
  path. 
  Coincident 
  with 
  the 
  cutting 
  of 
  the 
  

   Sappi 
  canyon, 
  but 
  at 
  a 
  slower 
  pace, 
  its 
  tributaries 
  were 
  cut 
  and 
  

   the 
  front 
  of 
  the 
  fan 
  was 
  developed 
  into 
  cliffs 
  by 
  headward 
  

   erosion 
  of 
  short, 
  steep, 
  wet-weather 
  streams 
  assisted 
  by 
  ground- 
  

   water 
  activities. 
  The 
  Ayahuaycco 
  probably 
  originated 
  as 
  a 
  

   line 
  of 
  drainage 
  at 
  the 
  western 
  edge 
  of 
  the 
  fan 
  and 
  worked 
  

   progressively 
  north 
  and 
  east, 
  following 
  the 
  rock 
  slope 
  down- 
  

   ward 
  and 
  maintaining, 
  approximately, 
  the 
  present 
  relation 
  of 
  

   one 
  bank 
  on 
  or 
  near 
  rock 
  and 
  the 
  other 
  bank 
  cut 
  in 
  gravel. 
  

  

  That 
  the 
  dissection 
  of 
  the 
  Pleistocene 
  and 
  recent 
  gravels 
  has 
  

  

  