﻿18 
  H. 
  E. 
  Gregory 
  — 
  Gravels 
  at 
  Cuzco. 
  

  

  Its 
  slope 
  is 
  200 
  / 
  per 
  mile 
  and 
  its 
  surface 
  is 
  diversified 
  by 
  flat- 
  

   tened 
  hills 
  much 
  reduced 
  by 
  grading, 
  valley 
  filling, 
  and 
  canal- 
  

   ization 
  of 
  streams, 
  processes 
  which 
  have 
  been 
  going 
  on 
  since 
  

   Inca 
  days. 
  The 
  upper 
  portion 
  of 
  the 
  fan 
  (fig. 
  1) 
  is 
  a 
  nearly 
  

   level 
  plateau, 
  deeply 
  trenched 
  by 
  the 
  Ayahuaycco 
  Quebrada,* 
  

   and 
  by 
  gravel-walled 
  canyons 
  tributary 
  to 
  the 
  Huatanay.f 
  

   The 
  Ayahuaycco 
  ravine 
  reaches 
  a 
  depth 
  exceeding 
  140'. 
  Its 
  

   northeast 
  wall, 
  cut 
  entirely 
  in 
  gravels 
  and 
  sands, 
  presents 
  slopes 
  

   of 
  20° 
  to 
  60° 
  which 
  increase 
  to 
  60° 
  to 
  80° 
  at 
  the 
  base 
  (see 
  fig. 
  1, 
  

   p. 
  6 
  ; 
  fig. 
  6, 
  p. 
  13) 
  ; 
  its 
  southwest 
  wall 
  is 
  of 
  gentler 
  slope, 
  and 
  

   consists 
  in 
  part 
  of 
  bed 
  rock. 
  The 
  five 
  western 
  tributaries 
  of 
  

   the 
  Huatanay 
  are 
  sharply 
  cut 
  gravel 
  canyons 
  ending 
  in 
  box 
  heads, 
  

   — 
  the 
  southernmost 
  one, 
  leading 
  to 
  a 
  flat 
  240' 
  above 
  the 
  river, 
  

   is 
  confined 
  between 
  banks 
  with 
  an 
  average 
  slope 
  of 
  70°, 
  and 
  at 
  

   one 
  point 
  presents 
  a 
  vertical 
  wall 
  110' 
  high. 
  In 
  fact 
  at 
  certain 
  

   points 
  in 
  these 
  canyons 
  the 
  walls 
  are 
  undercut 
  ten 
  to 
  fifteen 
  

   feet 
  without, 
  however, 
  interfering 
  with 
  the 
  stability 
  of 
  the 
  

   compact 
  gravel 
  mass. 
  Such 
  canyon 
  faces, 
  built 
  entirely 
  of 
  

   coarse 
  unconsolidated 
  sediments, 
  are 
  made 
  possible 
  by 
  the 
  dis- 
  

   tribution 
  of 
  calcareous 
  cement 
  among 
  the 
  finer 
  constituents, 
  

   and 
  by 
  exceptional 
  conditions 
  controlling 
  ground 
  water 
  circu- 
  

   ation. 
  To 
  reach 
  the 
  upper 
  gravel 
  flats 
  from 
  the 
  city 
  requires 
  

   an 
  ascent 
  of 
  400'. 
  

  

  Throughout 
  the 
  entire 
  area 
  covered 
  by 
  the 
  fan 
  gravels, 
  rock 
  

   is 
  exposed 
  only 
  in 
  the 
  bed 
  of 
  the 
  Huatanay 
  and 
  at 
  a 
  few 
  places 
  

   in 
  the 
  Ayahuaycco 
  and 
  its 
  tributaries. 
  (See 
  map, 
  fig. 
  2.) 
  

  

  Structure. 
  — 
  Speaking 
  broadly, 
  the 
  Cuzco 
  fan 
  is 
  built 
  of 
  

   thick, 
  widespread 
  deposits 
  of 
  gravel 
  within 
  which 
  are 
  included 
  

   lenses 
  of 
  fine 
  sand. 
  Most 
  of 
  the 
  gravels 
  are 
  very 
  coarse, 
  ap- 
  

   proximately 
  one-half 
  of 
  their 
  bulk 
  consisting 
  of 
  pebbles 
  exceed- 
  

   ing 
  an 
  inch 
  in 
  diameter. 
  Stratification 
  in 
  the 
  gravels 
  is 
  nowhere 
  

   well 
  developed, 
  but 
  may 
  be 
  detected 
  in 
  large 
  exposures 
  by 
  the 
  

   relatively 
  high 
  per 
  cent 
  of 
  flattened 
  pebbles 
  which 
  tend 
  to 
  

   assume 
  horizontal 
  positions. 
  However, 
  many 
  portions 
  of 
  the 
  

   quebrada 
  walls 
  1000-2000 
  square 
  feet 
  in 
  area 
  appear 
  equally 
  

   w 
  T 
  ell 
  stratified 
  vertically 
  and 
  horizontally 
  and 
  a 
  photograph 
  of 
  

   such 
  banks 
  reveals 
  essentially 
  the 
  same 
  structure 
  regardless 
  

   of 
  the 
  position 
  from 
  which 
  it 
  is 
  viewed. 
  (See 
  p. 
  1, 
  fig. 
  6.) 
  

   In 
  fact 
  the 
  difference 
  between 
  the 
  unmodified 
  fan 
  deposits 
  

   composed 
  exclusively 
  of 
  gravel 
  and 
  the 
  slides, 
  artificial 
  gravel 
  

   heaps, 
  or 
  the 
  bo 
  wider 
  y 
  floor 
  of 
  the 
  present 
  torrential 
  streams 
  

   is 
  to 
  be 
  detected 
  only 
  by 
  the 
  closest 
  scrutiny. 
  In 
  brief 
  a 
  large 
  

   part 
  of 
  the 
  materials 
  of 
  the 
  fan 
  constitute 
  a 
  heap 
  of 
  unassorted 
  

  

  * 
  Quebrada 
  is 
  a 
  Spanish-American 
  term 
  applied 
  to 
  narrow 
  steep-walled 
  

   water 
  courses 
  regardless 
  of 
  dimensions. 
  As 
  used 
  locally 
  the 
  term 
  includes 
  

   both 
  arroyos 
  and 
  canyon. 
  Ayahuaycco 
  signifies, 
  "Valley 
  of 
  the 
  Dead." 
  

  

  f 
  The 
  Huatanay 
  consists 
  of 
  the 
  Eio 
  Sappi 
  and 
  its 
  affluents. 
  

  

  