﻿254 
  REPORT 
  UNITED 
  STATES 
  GEOLOGICAL 
  SURVEY. 
  

  

  vaded 
  it 
  on 
  all 
  sides 
  and 
  the 
  edges 
  are 
  scalloped 
  by 
  a 
  thousand 
  cafi- 
  

   oned 
  gorges. 
  Not 
  only 
  have 
  the 
  destroying 
  forces 
  encroached 
  from 
  the 
  

   edges 
  on 
  all 
  sides, 
  but 
  the 
  mesa 
  has 
  been 
  entirely 
  severed 
  by 
  the 
  canon 
  

   of 
  the 
  Eio 
  Mancos, 
  which 
  cuts 
  through 
  it 
  from 
  north 
  to 
  south. 
  This 
  

   canon 
  sends 
  out 
  a 
  multitude 
  of 
  side 
  canons 
  and 
  gorges, 
  which 
  seem 
  lit- 
  

   erally 
  to 
  have 
  honeycombed 
  the 
  interior 
  of 
  the 
  tableland 
  ; 
  indeed, 
  when 
  

   we 
  come 
  to 
  look 
  more 
  closely 
  there 
  seems 
  onlj^ 
  the 
  skeleton 
  of 
  a 
  plateau 
  

   remaining 
  ; 
  this 
  is 
  well 
  shown 
  in 
  the 
  contour 
  map 
  (see 
  atlas 
  of 
  Colo- 
  

   rado). 
  The 
  relation 
  borne 
  by 
  the 
  Eio 
  Mancos 
  to 
  the 
  mesa 
  has 
  an 
  inter- 
  

   esting 
  bearing 
  upon 
  the 
  past 
  history 
  of 
  this 
  region. 
  Between 
  the 
  

   mesa 
  and 
  the 
  La 
  Plata 
  Mountains 
  there 
  is 
  a 
  broad 
  irregular 
  pas- 
  

   sage-way, 
  eroded 
  from 
  the 
  Middle 
  Cretaceous 
  shales. 
  The 
  river 
  is 
  

   formed 
  on 
  the 
  western 
  slopes 
  of 
  the 
  mountains, 
  crosses 
  this 
  low 
  belt 
  

   and 
  cuts 
  directly 
  into 
  the 
  northern 
  face 
  of 
  the 
  table, 
  which 
  presents 
  

   massive 
  cliifs 
  nearly 
  one 
  thousand 
  feet 
  in 
  height, 
  notwithstanding 
  the 
  

   fact 
  that 
  a 
  natural 
  passage-way 
  opens 
  out 
  both 
  east 
  and 
  west 
  between 
  

   the 
  mountains 
  and 
  table-land. 
  It 
  is 
  evident 
  that 
  the 
  river 
  could 
  not 
  

   have 
  taken 
  this 
  course 
  with 
  the 
  pi'esent 
  relations 
  of 
  surface-rock. 
  It 
  

   has 
  in 
  all 
  probability 
  been 
  "superimposed" 
  upon 
  the 
  present 
  face 
  of 
  the 
  

   country 
  by 
  having 
  been 
  first 
  laid 
  down 
  upon 
  a 
  surface 
  of 
  uniform 
  char- 
  

   acter. 
  Such 
  a 
  condition 
  of 
  surface 
  may 
  easily 
  have 
  existed 
  when 
  the 
  

   sandstones 
  of 
  the 
  Mesa 
  Verde 
  group 
  (or 
  of 
  any 
  higher 
  group) 
  extended 
  

   back 
  to 
  or 
  overlapped 
  the 
  trachytic 
  masses 
  of 
  the 
  mountains. 
  It 
  is 
  

   probable 
  that 
  when 
  the 
  bed 
  of 
  the.river, 
  in 
  its 
  descent 
  through 
  the 
  suc- 
  

   cessive 
  formations, 
  reached 
  the 
  shales, 
  the 
  cafion-wrflls 
  were 
  continuous 
  

   from 
  source 
  to 
  mouth, 
  and 
  consisted 
  of 
  the 
  massive 
  sandstones 
  of 
  the 
  

   Upper 
  Cretaceous 
  and 
  Tertiary 
  formations. 
  These 
  formations 
  would 
  

   probably 
  first 
  be 
  penetrated 
  along 
  the 
  base 
  of 
  the 
  mountains 
  where 
  

   considerable 
  uplifting 
  and 
  breaking 
  up 
  of 
  strata 
  had 
  occurred. 
  The 
  

   eroding 
  forces 
  on 
  reaching 
  the 
  soft 
  shale-beds 
  at 
  a 
  given 
  point 
  would 
  

   be 
  greatly 
  aided 
  by 
  a 
  process 
  of 
  undermining 
  by 
  which 
  the 
  cliffs 
  would 
  

   break 
  down 
  and 
  retreat 
  to 
  the 
  right 
  and 
  left, 
  forming 
  side 
  valleys, 
  and 
  

   these 
  in 
  time 
  would 
  meet 
  similar 
  lateral 
  valleys 
  from 
  the 
  McElmo 
  and 
  

   La 
  Plata 
  CaQons. 
  It 
  thus 
  seems 
  clear 
  that 
  the 
  course 
  of 
  the 
  Eio 
  Man- 
  

   cos 
  was 
  laid 
  out 
  at 
  a 
  very 
  early 
  period 
  and 
  that 
  the 
  formation 
  of 
  the 
  

   transverse 
  valley 
  separating 
  the 
  mesa 
  from 
  the 
  mountains 
  is 
  of 
  com- 
  

   paratively 
  recent 
  date. 
  

  

  At 
  the 
  entrance 
  of 
  the 
  caiion 
  on 
  the 
  north 
  the 
  cliffs 
  are 
  capped 
  by 
  the 
  

   sandstones 
  of 
  the 
  Lower 
  Escarpment, 
  which 
  are 
  highest 
  at 
  the 
  northern 
  

   edge 
  and 
  slope 
  off 
  toward 
  the 
  interior 
  of 
  the 
  mesa; 
  beneath 
  this 
  are 
  ex- 
  

   posed 
  700 
  or 
  800 
  feet 
  of 
  the 
  Middle 
  Cretaceous 
  shales. 
  The 
  dip 
  to 
  the 
  

   south 
  is 
  so 
  much 
  greater 
  than 
  the 
  fall 
  of 
  the 
  river 
  (see 
  section. 
  Plate 
  XL) 
  

   that 
  ten 
  miles 
  below 
  the 
  shales 
  disappear, 
  and 
  the 
  sandstones 
  of 
  the 
  

   Lower 
  Escarpment 
  outcrop 
  along 
  the 
  river-banks, 
  while 
  the 
  Coal-Meas- 
  

   ures 
  occupy 
  the 
  middle 
  slopes, 
  and 
  the 
  Upper 
  Escarpment 
  sandstones 
  

   cap 
  the 
  caHon- 
  walls. 
  The 
  general 
  height 
  of 
  these 
  walls 
  remains 
  quite 
  

   uniform, 
  being 
  measured 
  generally 
  by 
  the 
  thickness 
  of 
  the 
  Mesa 
  Verde 
  

   Group, 
  which 
  varies 
  from 
  1,000 
  to 
  1,200 
  feet. 
  

  

  Midway 
  in 
  the 
  caiion 
  the 
  Coal 
  Group 
  descends 
  to 
  the 
  river-bed, 
  but 
  

   below 
  this 
  the 
  dip 
  decreases 
  and 
  the 
  river 
  cuts 
  down 
  into 
  the 
  shales 
  

   again. 
  Throughout 
  the 
  portions 
  of 
  the 
  caiion 
  in 
  which 
  the 
  firm 
  strata 
  

   of 
  the 
  Mesa 
  Verde 
  Group 
  only 
  occur, 
  the 
  walls 
  are 
  steep 
  and 
  close. 
  

  

  The 
  Coal 
  series 
  is 
  always 
  prominent 
  ; 
  the 
  dark 
  lines 
  of 
  the 
  outcrops 
  

   appear 
  everywhere 
  int 
  he 
  middle 
  portions 
  of 
  the 
  walls. 
  The 
  prepon- 
  

   derance 
  of 
  sandstones 
  is 
  very 
  noticeable, 
  and 
  beds 
  of 
  great 
  thickness 
  

   appear 
  in 
  all 
  parts 
  of 
  the 
  section. 
  These 
  beds 
  are 
  exceedingly 
  variable, 
  

  

  