Relative age dating definition
You can't start with the top layer and add the ones under it since there would be nothing to build on.Principle of later continuity: The principle of lateral continuity says that rocks continue in all lateral directions, even if you can't see them on the surface.This means that a quartz sandstone deposited 500 million years ago will look very similar to a quartz sandstone deposited 50 years ago.Making this processes even more difficult is the fact that due to plate tectonics some rock layers have been uplifted into mountains and eroded while others have subsided to form basins and be buried by younger sediments.
Although there might be some mineral differences due to the difference in source rock, most sedimentary rock deposited year after year look very similar to one another.
So, basically it's the difference between saying 'I'm 25 years old, and my sibling is 20 years old' and 'I'm older than my sibling.' Geologists use a variety of techniques, or principles, to determine the relative age of a rock unit.
Principle of original horizontality: This principle simply states that deposition of rocks occurs horizontally or nearly horizontally. Most deposition occurs as the ocean deposits sediment flat and horizontally on existing rock, either on the ocean floor or on the continent.
The most obvious feature of sedimentary rock is its layering.
This feature is produced by changes in deposition over time.
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To understand this better, consider a four-layer cake. It wouldn't be possible for the other layers to be added without the bottom layer first.