I'm going to focus on the oft-cited Grand Canyon here not because it's the only major evidence against a global flood, but because young-earth creationists virtually universally agree that the Grand Canyon was created in the still-soft sedement from the Flood. As such I will make a couple of assumptions about the young-earth creationist position:
First, the Grand Canyon is proposed to have been carved in rather soft sedement during or shortly after the great flood.
Second, the layers in which the Grand Canyon is cut are proposed to have all been laid down by the flood itself.
If you (as a young-earth creationist reader) object to these assumptions, I'd ask first why no major young-earth creationist organization has objected to these claims. In short, do you have citations for your objections, or did you come up with them yourself? If you really intend to counter by rejecting these assumptions, please explain how ANY amount of water could cut such a narrow channel in the many layers of solid rock in under a year (or even two years to be generous). Please show your work. And do forward it to AiG, ICR and any other creationist you can think of so you're all working on the same page!
Given that the young-earth creationist position assumes these two things, I will go over the top 1000 ft of rock (out of the 5000 ft of the Grand Canyon at it's deepest). Note that precambrian rocks do not start until 3000 ft down, so I'm still well within laptoppop's proposed flood-deposition range. Further, as the Colorado River cannot flood over these 5000 ft (!) and there is no other mechanism for deposition on the scale of inches (much less thousands of feet) I think it would be safe to assume that these top 1000 feet haven't been deposited in the last 4000 years.
The top four layers (going down 1000 ft) from bottom to top are Hermit Shale, Coconino Sandstone, Toroweap Formation, and Kaibab Formation.
Below the Hermit Shale are a series of coastal layers deposited at various oceanic depths. There are many layers that were alternately at the bottom of the ocean or on the coast (never deeper than a shallow marine shelf).
the Hermit Shale was a coastal swamp as evidenced by its extensive preservation of terrestrial plants, insect wings etc... Interestingly enough, the layers just below the Hermit Shale were apparently laid down as a coastal plain, though the layers below THAT were definitely marine. Apparently the flood stopped long enough for the plain to become populated with a variety of grasses and animals before converting back to an intertidal shallow marine shelf, and then a swamp?
Next is the Coconino Sandstone layers -- windblown sand dunes that stretched as far as Montana! This is quite a desert (much like the Sahara) and could hardly have dried out and generated dunes in the year of a global flood! Sometimes creationists claim that the dunes were underwater -- but this is not supported by the numerous reptillian tracks -- some as big as a cow! Since this is over 4000 ft from where the flood must have started depositing, this reptillian creature must have been treading water for months before making tracks in the sand! Other tracks like those of centepede-like insects and raindrop impressions are also found discounting claims of underwater deposition (in which you would find none of this, with plenty of fish scales, and aquatic, not land plants.
Above the Coconino Sandstone is the Toroweap Formation. This is again near the shore, but in this layer there are the dessecated formations I mentioned in an earlier thread. Quite simply, there are dried-out, dessecated salt flats with crystals that cannot form on the bottom of the ocean. They are formed by long periods of evaporation. Of course, the Toroweap Formation is primarily costal, but the existance of such extensive salt flats presents a bit of a problem with any proposed global flood.
Finally, the Kaibab Formation -- an off-shore shallow marine shelf. There are plenty of marine fossils in here, including coral, mollusks, fish teeth etc... If the Grand Canyon were cut prior to this layer, there would be evidence of the reflooding SOMEWHERE along the canyon -- particularly in places where the Canyon was rerouted leaving channels cut into the rock, yet without further erosion to remove evidence of the reflooding. That shows that the earlier salt flats in the Toroweap formation must have beem created prior to the formation of the Grand Canyon -- again bringing up the question, "how was there a vast desert followed by multiple dessecation events during a year-long global flood?!?"
I'm afraid I've lost most readers' attention by now, and I've nowhere NEAR exhausted the information out there on these layers. And remember, this is only 1/5 of the depth of the Grand Canyon! I don't fault people for avoiding the massive heaps of dry and often boring information out there on the geologic column, but if avoiding the information, I rather feel one should not claim to hold truth on the subject.
A few quick sources, although I have to admit that about half of this post was from memory, and although I checked all the major facts, if I were to go for more sources, I'd end up writing a book, not a forum post!
http://www.kaibab.org/geology/gc_layer.htm
http://www.rockhounds.com/grand_hikes/geology/overview.shtml
http://www.astro.lsa.umich.edu/~cowley/GCandMoon.html
http://www.talkorigins.org/faqs/icr-visit/bartelt4.html
