﻿92 
  

  

  NEW 
  YORK 
  STATE 
  MUSEUM 
  

  

  that 
  it 
  disappears 
  by 
  thinning, 
  either 
  bringing 
  the 
  layers 
  above 
  and 
  

   below 
  it 
  in 
  contact 
  or 
  giving 
  way 
  to 
  a 
  bed 
  of 
  shale. 
  

  

  A 
  careful 
  examination 
  of 
  these 
  individual 
  beds 
  will 
  show 
  the 
  pres- 
  

   ence 
  of 
  ripple 
  marks 
  in 
  many 
  of 
  them. 
  This 
  indicates 
  moderately 
  

   shallow 
  water 
  during 
  the 
  accumulation 
  of 
  these 
  sands; 
  for 
  ripple 
  

   marks 
  are 
  found 
  only 
  down 
  to 
  the 
  depth 
  to 
  which 
  wave 
  action 
  pene- 
  

   trates. 
  These 
  ripples 
  vary 
  greatly 
  in 
  size, 
  a 
  bed 
  about 
  lo 
  feet 
  above 
  

   the 
  concretionary 
  layer 
  showing 
  examples 
  in 
  which 
  the 
  crests 
  are 
  

   from 
  one 
  to 
  one 
  and 
  a 
  half 
  or 
  more 
  feet 
  apart. 
  

  

  The 
  fossils 
  found 
  in 
  these 
  sandstones 
  are 
  the 
  characteristic 
  Medina 
  

   pelecypods, 
  and 
  the 
  common 
  Medina 
  Lingula 
  cuneata. 
  

  

  5 
  The 
  thin 
  bedded 
  sandstone 
  layers 
  are 
  followed 
  by 
  12 
  or 
  15 
  feet 
  

   of 
  massive 
  sandstones 
  in 
  beds 
  from 
  one 
  to 
  several 
  feet 
  in 
  thickness, 
  

   and 
  varying 
  in 
  color 
  from 
  reddish 
  to 
  grayish. 
  This 
  rock 
  generally 
  

   shows 
  strongly 
  marked 
  cross-bedding 
  structure 
  on 
  those 
  faces 
  

  

  

  

  Fig. 
  20a 
  Cross-bedding 
  iu 
  Medina 
  sandstone, 
  Niagara 
  gorge. 
  

  

  which 
  have 
  been 
  exposed 
  for 
  some 
  time. 
  This 
  structure 
  illustrated 
  

   in 
  figure 
  20a, 
  copied 
  from 
  a 
  ledge 
  of 
  this 
  rock, 
  indicates 
  diverse 
  cur- 
  

   rent 
  and 
  wave 
  action 
  in 
  the 
  shallow 
  water 
  in 
  which 
  this 
  rock 
  was 
  

   forming. 
  While 
  the 
  deposition 
  of 
  the 
  strata 
  was 
  essentially 
  hori- 
  

   zontal, 
  the 
  minute 
  layers 
  made 
  up 
  of 
  the 
  sand 
  grains 
  were 
  for 
  a 
  

   time 
  deposited 
  at 
  a 
  high 
  angle, 
  much 
  after 
  the 
  manner 
  of 
  deposition 
  

   of 
  the 
  layers 
  in 
  a 
  delta. 
  After 
  a 
  while 
  the 
  activity 
  of 
  the 
  current 
  

   changed 
  to 
  another 
  direction, 
  and 
  the 
  layers 
  already 
  deposited 
  were 
  

   in 
  part 
  eroded, 
  or 
  beveled 
  across 
  the 
  top, 
  and 
  new 
  layers, 
  inhar- 
  

  

  