Questions for evolution believers - the geological column (page 2)

Qcmbr

So just to recap:

Layering is found in the igneous rocks which are generally laid down in big masses and are dateable by radio carbon dating, the sedimentary layers are made from eroded material and so can't be dated by these methods - their ages have to be sought using different measures - and its not too accurate. The fossils appear in the sedimentary layers.

Now to me we have a another question - to fossilise an animal as big as say a TRex you need to cover them up pretty fast with loads of silt otherwise they rot and decompose (e.g. of the millions of buffalo and bisen that died over the years there aren't (m)any? fossil remains. Fossilisation needs to be fast as well since even being buried in mud isn't itself enough - has to be anerobic or chemically nasty to bacteria etc..? Are we saying then that fossilisation occurs over millions of years of gradual deposition or are sediment layers laid down in fast pulses (maybe a local flood) which cover up these animals and exclude bacteria...or can really arid areas produce sedimentary rocks?

AlmostAtheist

Here's a sadly simplistic page about how fossils are formed. While simplistic, it does make it obvious that fossilization should be a rare event. It's a kids page, so forgive the "Mr. McClure, how do fossils form? Well, Jimmy..." approach.

Dave

Qcmbr

So big animals need to be covered pretty quickly - does this argue for a very different earth to that which we see today - ie extremely cataclysmic conditions wherever fossils are found? Shouldn't there be lots of other things fossilised with the dinosaur (ie what it was lying on?) - what was in its stomach - if it had bones?

Aztec

Woohoo! Enchanted Learning sounds more my speed!

AlmostAtheist

There are examples of fossilized stomach contents:
http://www.oceansofkansas.com/plesio4.html

And dinosaur nests:
http://www2.worldbook.com/wc/popup?path=features/dinosaurs&page=html/nests.htm&direct=yes

Qcmbr

How does coal fit into this gelogical column - coal seems (haha) to be very concentrated and fairly substantial - what conditions created coal since they can't have been depositional like rock - I'm fairly sure there are no coals being formed today - is that correct?

AlmostAtheist

No insult intended, but it's another kid's site. It points out that the coal-making process continues today, but it's so tremendously slow that it can't be at all evident.

http://kids.rrc.state.tx.us/school/classroom/coal/coalfrom.html

Qcmbr

..so is oil halfway coal? I would have though coal would appear at every level of the geological column and be at different stages of production. Even if teh majority is 300 million years old - wheres teh 100 million year old midway coal?

tetrapod.sapien

:So big animals need to be covered pretty quickly

not so. fossilization can occur long after the animal has fully decomposed, and all that is left are bones. actually this is how pretty much all human ancestor fossils are found. some smaller dinosaurs (and aquatic exoskeletal organisims like trilobites and fish) are often fossilized by being drowned in mud in some event, but the larger ones, like hominid fossils are usualy found scattered in a small area.

AlanF

Aztec, you're smart enough that you don't need to be led by the hand through these things. Besides, you learn better by doing your own research.

Qcmbr, you're not reading carefully.

: Layering is found in the igneous rocks

I said exactly the opposite.

: which are generally laid down in big masses

Good!

: and are dateable by radio carbon dating,

I said the opposite. I said that radiometric dating is appropriate for dating rocks, and gave a couple of examples.

: the sedimentary layers are made from eroded material and so can't be dated by these methods - their ages have to be sought using different measures

Right.

: - and its not too accurate.

I didn't say that. I said that they are sometimes dateable. I said nothing about accuracy.

: The fossils appear in the sedimentary layers.

Good!

: Now to me we have a another question - to fossilise an animal as big as say a TRex you need to cover them up pretty fast with loads of silt otherwise they rot and decompose

Your phrasing leaves much to be desired in terms of showing that you understand what's going on. Most of the time when fossilization takes place, the flesh of a dead large animal rots completely away while the carcass is lying on the surface, and then the bones are buried. For example, in Dinosaur National Monument in Utah, there's a big accumulation of large dinosaur skeletons. The critters apparently died by various means at various times, the carcasses were washed down a small river and accumulated on a sandbar, then for whatever reason the entire area was suddenly buried in enough sediment to cover the whole assemblage. The whole burial process, from beginning to end, clearly took some time, since many of the bones have tooth marks on them due to gnawing by scavengers. Eventually the region was covered by a shallow sea, and buried in thick sediments.

This kind of process can be seen in desert areas today. For example, in the Sonoran Desert in Arizona, the climate until about 1880 was such that the flat desert areas were gradually accumulating sediments. These sediments were from the occasional but massive flooding that inundated large areas every once in a while. Today, there is net erosion, and you can see the thick accumulations of flat-lying sediments where road cuts expose them. Obviously, any large skeletons lying on the desert ground would have been buried by these floods.

Also, fossils are often found in sandstone laid down by wind rather than silt. For example, in the early 1970s in Mongolia there were found the entangled fossils of a Velociraptor and a Protoceratops, buried in sandstone that was clearly the remains of sand dunes. The animals had clearly fought and killed each other, and then been buried. So burial and fossilization can take place in water or on the land.

: (e.g. of the millions of buffalo and bisen that died over the years there aren't (m)any? fossil remains.

Right, but there are a few. For example, I'm pretty sure that the La Brea Tar Pits in Los Angeles contain the remains of ancient bison. There are also accumulations of bison skeletons where a herd ran off a cliff and died en masse. Most of these are due to the hunting practices of Native Americans, such as at the Canadian national monument called Head Smashed In Buffalo Jump in Alberta. The frozen remains of an extinct bison were found in the late 1970s in thawing permafrost in Alaska. The beast had been killed and largely eaten by lions, then frozen, then gradually buried in creeping mud flows, some 35,000 years ago.

: Fossilisation needs to be fast as well since even being buried in mud isn't itself enough - has to be anerobic or chemically nasty to bacteria etc..?

Right. Occasionally, burial in silt or sand or mud produces just the right conditions to inhibit rotting of the bones. Burial in dry sand acts in the same way that natural mummies are produced. There isn't anything for the bacteria to live on, and the bones quickly dry out. After that, nothing can produce rot. Fossilization of fleshy parts is extremely rare.

: Are we saying then that fossilisation occurs over millions of years of gradual deposition

No.

: or are sediment layers laid down in fast pulses (maybe a local flood)

Sometimes. It depends on which fossilization process you're talking about. See above for examples.

: which cover up these animals and exclude bacteria...

Sometimes.

: or can really arid areas produce sedimentary rocks?

Yes.

AlanF

Questions for evolution believers - the geological column

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