Law of crosscutting relationships

[kingwinner]

Hello, I am back...haven't posted a question for a long time...

1.

"The Eocene rocks are older than the 57 m.y. old dike and younger than the 36 m.y. old dike that cuts through them."

http://www.earthsci.org/teacher/basi...im/geotim.html

WHY? I don't get the quoted part from the web site above, can someone explain?



2) For the law of crossingcutting relationships, it states that an intrustion or a fault it younger than the layers it cuts through. However, does this law also implies that the fault or intrusion is older than all the layers that it doesn't cut through? (This is the main thing that drew me into confusion)

So, say if a fault is cut off below an unconformity, how can you use this law alone to tell the relative age of the fault and of the rock layers that were deposited above the unconformity?

I would say that the fault is younger than all layers below the unconformity, but what else can I say based on the above record? How about the age of the layers above the unconformity in relation to the age of the fault, can I really tell?

I really need some explanations to clear up these concepts! Thanks!

[LOC]

Originally Posted by kingwinner

1.

"The Eocene rocks are older than the 57 m.y. old dike and younger than the 36 m.y. old dike that cuts through them."

http://www.earthsci.org/teacher/basi...im/geotim.html

WHY? I don't get the quoted part from the web site above, can someone explain?

The statement is wrong. Interchange the words "younger" and "older."

2) For the law of crossingcutting relationships, it states that an intrustion or a fault it younger than the layers it cuts through. However, does this law also implies that the fault or intrusion is older than all the layers that it doesn't cut through? (This is the main thing that drew me into confusion)

Faults and intrusions are younger than the layers they cut and any other rocks the same age or older.

So, say if a fault is cut off below an unconformity, how can you use this law alone to tell the relative age of the fault and of the rock layers that were deposited above the unconformity?

A fault cut by an unconformity is younger than the rocks it cuts, older than the unconformity that cuts it, and older than all the rocks deposited above the unconformity.

I would say that the fault is younger than all layers below the unconformity, but what else can I say based on the above record? How about the age of the layers above the unconformity in relation to the age of the fault, can I really tell?

See above.

Draw it out on a piece of paper if you can't imagine it.

I really need some explanations to clear up these concepts! Thanks!

I think you just need to think on them a bit longer.