A polystrate tree fossil is a fossilized tree that protrudes through multiple layers of strata. Here is a picture of one.
These have been found in many places on Earth.
Here’s how a typical creationist web site argues that this is evidence for a young Earth:
“It is not possible that polystrate fossils were buried gradually over many thousands or hundreds of thousands of years because the top part of any tree would have rotted away before it could be protected by sediment. Polystrate fossils point to rapid burial and are evidence for the reality of the global Flood recorded in the Bible.”
It would seem to be a persuasive argument.
Except for the “global” part of the claim.
It’s surely true that floods probably contribute to these fossils. But it doesn’t take a global flood to fossilize a single tree. A single local flood can do that.
Moreover even several feet of sediment from a flood rarely kills a tree. It will continue to grow. Eventually when it dies, it fossilizes and turns into a polystrate fossil.
Floods, yes. Global flood, no.
 http://creationontheweb.com/content/view/5894/ referenced on February 23, 2009
 http://www.talkorigins.org/faqs/polystrate/trees.html, referenced on February 23, 2009