How Are Fossilized Rocks Like This Leaf Formed?

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Jimmy Snyder
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A friend and colleague found a fossil and brought it into show me. It is a small leaf in a rock. He asked me how the rock was formed, but I couldn't say as I know nothing of such things. The rock is layered. You can see the various layers by looking at the rock edgewise. In fact, the edge is so ragged that you can see fossils of leaves in several places. It looks like you could peel the layers of the rock. Indeed, the ragged edge shows that the bonds in one of the axes are weaker than those in the other two. I guess this is sedementary rock formed by layers of silt building up and then compressed into hardness. The rock is quite hard, not like compacted silt, but more like melted and recongealed silt.

My question is what could cause the silt to form such rigid bonds in the horizontal directions and such weak ones in the vertical direction? Or have I got the whole picture messed up?

Edit- Now that I think about it a little more, the bonds in the vertical direction are just as strong as the ones in the horizontal direction. The rock can only be split at certain places. Perhaps 50 per inch or so. It's as if the sedement collected and hardened before the next layer was deposited. But what could make it harden?
 
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You are correct. It is sedimentary rock - highly layered.

Wiki seems to have a pretty good description of the process.
 
DaveC426913 said:
You are correct.
It's a rare pleasure.

Which article should I look at?
 
Fossils are so cool, i have some of tiny sea creatures about 2mm, and i can still see patterns in the shells, to think they are millions of years old, i value them more than
most things.
 
jimmysnyder said:
A friend and colleague found a fossil and brought it into show me. It is a small leaf in a rock. He asked me how the rock was formed, but I couldn't say as I know nothing of such things. The rock is layered. You can see the various layers by looking at the rock edgewise. In fact, the edge is so ragged that you can see fossils of leaves in several places. It looks like you could peel the layers of the rock. Indeed, the ragged edge shows that the bonds in one of the axes are weaker than those in the other two. I guess this is sedementary rock formed by layers of silt building up and then compressed into hardness. The rock is quite hard, not like compacted silt, but more like melted and recongealed silt.

My question is what could cause the silt to form such rigid bonds in the horizontal directions and such weak ones in the vertical direction? Or have I got the whole picture messed up?

Edit- Now that I think about it a little more, the bonds in the vertical direction are just as strong as the ones in the horizontal direction. The rock can only be split at certain places. Perhaps 50 per inch or so. It's as if the sedement collected and hardened before the next layer was deposited. But what could make it harden?

Chances are, by your description, it's a siltstone. Check out lithification and diagenesis on wiki, although not sure if these pages are what Dave was referring to. Also, about bonding, the wekest bonds will in general be between bedding planes, which gives the rock a bulk strength which is weakest in the direction parallel to the bedding planes; you might also notice that it prefers to break at an angle (perhaps 60 degrees) from the bedding, check out cleavage.
 
jimmysnyder said:
It's a rare pleasure.

Which article should I look at?

'sedimentary rock' would be good