The famous Green River Formation (including shale and limestone) 
  covers tens of thousands of square miles. In at least one place, 
  it contains about twenty million varves, each varve consisting of 
  a thin layer of fine light sediment and an even thinner layer of 
  finer dark sediment. According to the conventional geologic 
  interpretation, the layers are sediments laid down in a complex of 
  ancient freshwater lakes. The coarser light sediments were laid 
  down during the summer, when streams poured run-off water into the 
  lake. The fine dark sediments were laid down in the winter when 
  there was less run-off. (This process can be observed in modern 
  freshwater lakes.) If this interpretation is correct, the varves 
  of the Green River formation must have formed over a period of 
  about twenty million years.
  
  Creationists insist that the earth is no more than 10,000 years 
  old and that the geologic strata were laid down by the Flood. 
  Whitcomb and Morris therefore attempt to attribute the Green River 
  varves to "a complex of shallow turbidity currents..." (p. 427). 
  Turbidity currents--flows of mud-laden water--generally occur in 
  the ocean, resulting from underwater landslides. If the Green 
  River shales were laid down during the Flood, there must have been 
  forty million turbidity currents, alternatively light and dark, 
  over about three hundred days. A simple calculation (which 
  Creationists have avoided for twenty years) shows that the layers 
  must have formed at the rate of about three layers every two 
  seconds. A sequence of forty million turbidity currents covering 
  tens of thousands of square miles every two-thirds of a second 
  seems unlikely.
  
  Henry Morris apparently has no answer to this. Biologist Kenneth 
  Miller of Brown University dropped this bombshell on Morris during 
  a debate in Tampa, Florida, on September 19, 1981, and Morris 
  didn't attempt a reply. Fred Edwords used essentially the same 
  argument against Duane Gish in a debate on February 2, 1982, at 
  the University of Guelph, Ontario. In rebuttal, Gish claimed that 
  some of the fossilized fishes project through several layers of 
  sediment and that therefore the layers can't be semiannual.
  
  As usual, Gish's argument ignores the main issue, which is the 
  alleged formation of millions of distinct layers of sediment in 
  less than a year. Furthermore, Gish's argument is false, according 
  to American Museum of Natural History paleontologist R. Lance 
  Grande, an authority on the Green River Formation. Grande says 
  that, while bones or fins of an individual fish may cut several 
  layers, in general each fish is blanketed by a single layer of 
  sediment. The few exceptions are explainable when one observes 
  lakes where varves are forming _today_. It sometimes happens that 
  a dead fish is too large to be covered by one semiannual 
  sedimentation, and so its bones or fins end up protruding through 
  newer layers that are later observed to form. When an object or 
  animal is too large, this _must_ happen, and therefore such a 
  protrusion cannot be used as evidence against a great age for the 
  Green River Formation.
  
    --Robert Schadewald, "Six 'Flood' Arguments Creationists
      Can't Answer," _Creation Evolution_, Issue IX, pp. 12-17
     (1982).

The layers of sediment in this formation stretch undisturbed for many 
miles. There are 40 million layers in one part of this formation. All
deposited within a year, without any disturbance such as would be expected
from a "catastrophic flood"? Two remarkably thin, uniform, and miles-wide
layers every three seconds? Turbidity currents? Give me a break! 

My challenge for Creationists to explain the Green River Formation as
having been formed during the Flood still stands.