They complicate the task of relative dating, because they don't give an accurate picture of what happened in geologic history.

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.

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.

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.Let's say, in this set of rock strata, that we found a single intrusion of igneous rock punching through the sedimentary layers.We could assume that this igneous intrusion must have happened after the formation of the strata.Geologists find the cross-cutting principle especially useful for establishing the relative ages of faults and igneous intrusions in sedimentary rocks.Sometimes, geologists find strange things inside the strata, like chunks of metamorphic or igneous rock.Your goal is to study the smooth, parallel layers of rock to learn how the land built up over geologic time.