﻿H. 
  E. 
  Gregory 
  — 
  Gravels 
  at 
  Cuzco. 
  27 
  

  

  slides 
  like 
  an 
  artificial 
  revetment. 
  The 
  northeast 
  wall 
  is 
  

   marked 
  by 
  two 
  slides, 
  one 
  immediately 
  above 
  and 
  one 
  immedi- 
  

   ately 
  below 
  the 
  locality 
  from 
  which 
  bones 
  were 
  excavated 
  by 
  

   the 
  expedition 
  of 
  1911. 
  Moreover 
  this 
  wall 
  is 
  capped 
  by 
  a 
  

   sloping 
  terrace 
  with 
  an 
  escarpment 
  at 
  its 
  inner 
  face, 
  indicating 
  

   that 
  a 
  mass 
  of 
  gravel 
  30x300x150 
  feet 
  has 
  slid 
  downward 
  

   ten 
  feet 
  toward 
  the 
  valley 
  axis. 
  An 
  examination 
  of 
  old 
  slides 
  

   as 
  well 
  as 
  of 
  those 
  which 
  have 
  occurred 
  within 
  the 
  last 
  few 
  

   years 
  shows 
  that 
  except 
  where 
  decomposed 
  rock 
  forms 
  the 
  

   slipping 
  plane 
  there 
  is 
  no 
  clean 
  break 
  between 
  the 
  gravel 
  in 
  

   place 
  and 
  the 
  transported 
  portion 
  or 
  between 
  different 
  portions 
  

   of 
  the 
  slide 
  material 
  itself. 
  Where 
  sand 
  or 
  adobe 
  lenses 
  have 
  

   been 
  involved 
  it 
  is 
  easy 
  to 
  determine 
  botli 
  the 
  fact 
  and 
  the 
  

   amount 
  of 
  displacement, 
  but 
  in 
  the 
  massive 
  gravel 
  where 
  evi- 
  

   dence 
  of 
  stratification 
  is 
  absent, 
  it 
  is 
  impossible 
  to 
  determine 
  

   with 
  assurance 
  which 
  is 
  slide 
  and 
  which 
  is 
  original 
  bank, 
  espe- 
  

   cially 
  after 
  the 
  low 
  top 
  escarpment 
  has 
  been 
  obliterated 
  by 
  

   further 
  sliding 
  and 
  soil 
  creep. 
  Two 
  artificial 
  trenches 
  were 
  

   cut 
  into 
  gravels 
  across 
  a 
  plane, 
  which 
  on 
  independent 
  evidence 
  

   is 
  known 
  to 
  mark 
  the 
  contact 
  of 
  slide 
  gravel 
  with 
  the 
  original 
  

   valley 
  wall. 
  In 
  these 
  cases 
  neither 
  the 
  orientation 
  of 
  pebbles, 
  

   nor 
  a 
  ragged 
  contact, 
  nor 
  open 
  spaces, 
  nor 
  effects 
  of 
  ground 
  

   water 
  gave 
  evidences 
  of 
  displacement. 
  A 
  tunnel 
  eleven 
  feet 
  

   long 
  and 
  with 
  a 
  cross 
  section 
  4-JX3 
  feet 
  was 
  sunk 
  into 
  the 
  

   steep 
  gravel 
  wall 
  of 
  the 
  Ayahuaycco 
  Quebrada 
  at 
  the 
  exact 
  

   point 
  from 
  which 
  human 
  bones 
  were 
  taken 
  by 
  the 
  members 
  of 
  

   the 
  Peruvian 
  Expedition 
  of 
  1911. 
  The 
  section 
  exposed 
  is 
  

   entirely 
  gravel, 
  consisting 
  of 
  pebbles 
  of 
  brown 
  sandstone 
  (90 
  

   per 
  cent), 
  gray 
  sandstone 
  (9 
  per 
  cent), 
  limestone 
  and 
  igneous 
  

   fragments 
  (1 
  per 
  cent), 
  ranging 
  in 
  size 
  from 
  one-half 
  inch 
  to 
  

   four, 
  rarely 
  six 
  inches, 
  and 
  so 
  firmly 
  packed 
  that 
  no 
  timbering 
  

   of 
  the 
  tunnel 
  was 
  required. 
  The 
  material 
  is 
  uniform 
  in 
  tex- 
  

   ture, 
  contains 
  no 
  bands 
  or 
  lenses 
  of 
  sand, 
  no 
  division 
  planes, 
  or 
  

   other 
  unmistakable 
  evidences 
  of 
  stratification. 
  Even 
  the 
  peb- 
  

   bles 
  are 
  variously 
  oriented 
  and 
  about 
  a 
  third 
  of 
  % 
  them, 
  includ- 
  

   ing 
  many 
  thin, 
  flat 
  slabs, 
  slope 
  at 
  angles 
  between 
  60° 
  and 
  90° 
  

   to 
  a 
  horizontal 
  surface. 
  The 
  relation 
  of 
  landslides 
  to 
  the 
  

   gravels 
  in 
  which 
  human 
  bones 
  were 
  found 
  by 
  the 
  Expedition 
  of 
  

   1911 
  is 
  discussed 
  by 
  Doctor 
  Eaton 
  (see 
  p. 
  5) 
  and 
  need 
  not 
  be 
  fur- 
  

   ther 
  considered 
  here. 
  In 
  certain 
  of 
  the 
  slides 
  the 
  gravel 
  pebbles 
  

   appear 
  to 
  have 
  moved 
  differentially 
  among 
  themselves, 
  to 
  have 
  

   assumed 
  an 
  angle 
  of 
  repose 
  by 
  internal 
  readjustment, 
  somewhat 
  

   analogous 
  to 
  the 
  movement 
  of 
  particles 
  involved 
  in 
  glacier 
  

   motion. 
  It 
  would 
  therefore 
  appear 
  impracticable 
  to 
  determine 
  

   the 
  position 
  and 
  dimensions 
  of 
  landslides 
  and 
  slumps 
  in 
  the 
  

   unassorted 
  portions 
  of 
  the 
  Cuzco 
  gravels. 
  Evidence 
  of 
  dis- 
  

   placement 
  indicates 
  the 
  presence 
  of 
  landslides, 
  but 
  unfortu 
  

   nately 
  the 
  absence 
  of 
  such 
  evidence 
  does 
  not 
  prove 
  the 
  absence 
  

   of 
  landslides 
  or 
  " 
  creep 
  " 
  at 
  any 
  given 
  point. 
  

  

  