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It's called the Principle of Original Horizontality, and it just means what it sounds like: that all rock layers were originally horizontal. As you can imagine, regular sediments, like sand, silt, and clay, tend to accumulate over a wide area with a generally consistent thickness.It sounds like common sense to you and me, but geologists have to define the Principle of Original Horizontality in order to make assumptions about the relative ages of sedimentary rocks. Say you have a layer of mud accumulating at the bottom of a lake. More sediment accumulates from the leaf litter and waste of the forest, until you have a second layer.
Once we assume that all rock layers were originally horizontal, we can make another assumption: that the oldest rock layers are furthest toward the bottom, and the youngest rock layers are closest to the top. The forest layer is younger than the mud layer, right? When scientists look at sedimentary rock strata, they essentially see a timeline stretching backwards through history.
The highest layers tell them what happened more recently, and the lowest layers tell them what happened longer ago.
Free 5-day trial Discover how geologists study the layers in sedimentary rock to establish relative age.
Learn how inclusions and unconformities can tell us stories about the geologic past.
Since we assume all the layers were originally horizontal, then anything that made them not horizontal had to have happened after the fact.
We follow this same idea, with a few variations, when we talk about cross-cutting relationships in rock.
Let's say we find out, through numerical dating, that the rock layer shown above is 70 million years old.
We're not so sure about the next layer down, but the one below it is 100 million years old. Not exactly, but we do know that it's somewhere between 70 and 100 million years old.
Geologists establish the age of rocks in two ways: numerical dating and relative dating.
Numerical dating determines the actual ages of rocks through the study of radioactive decay.
Your goal is to study the smooth, parallel layers of rock to learn how the land built up over geologic time.Tags: Adult Dating, affair dating, sex dating