Most sedimentry rocks are originally deposited in shallow seas. For example,clasts carried down by rivers are deposited as beds, with brakes in deposition showing up as bedding planes . These beds are commonly laid down horizontally or very close to horizontal. It is therfeore assumed that if layers of rock are tilted, then they have moved from this original position.
The Principle Of Superposition
The principle states that the rocks at the bottom of a sequence are always the oldest and younger rocks are laid down on top of older ones. for example, rocks at the bottom of the cliff are older than those at the top. This assumes the rock have not been tilted upside down.
Way-up criteria
These structures are only formed one way up and so if these are present we can tell if the rocks have been turned upside down.
- Desication cracks
- Graded bedding
- Cross bedding
- Fossils can sometimes be found in life position and so indicate the way they lived
Fragments eroded from an older rock can be found within younger rock. The fragments have to be older than the rock they are found in.
e.g's
- Xenoliths found in igneous rocks have to be older, as they are fragments of country rock that fell into the magma due to stoping.
- Derived fossils are also older than the sediment they are found within. they have been eroded from older beds and redeposited in younger beds
- Pebbles in a conglomerate are older rocks eroded then redeposited
Cross- cutting relationships
Features which cut through rocks must be younger thanthe rocks they cut in. An example would be a dyke, which cuts through sediments. The sediments had to be there first for the dyke to be able to intrude them. Similary structures such as faults that cross-cut strata are, by definition younger than the beds they cut.
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