With this in mind geologist have long known that the deeper a sedimentary rock layer is the older it is, but how old?
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.
Relative time places events or formations in order based on their position within the rock record relative to one another using six principles of relative dating.
Relative time can not determine the actual year a material was deposited or how long deposition lasted; it simply tell us which events came first.
Using this process geologists are able to assign actual ages with known degrees of error to specific geologic events.
By combining knowledge gained using both relative and absolute dating processes geologists have been able to produce the geologic time scale.
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.
The difference in time between the youngest of the Proterozoic rocks and the oldest of the Paleozoic rocks is close to 300 million years.
The lava flow took place some time after the diorite cooled, was uplifted, and then eroded.
(Hammerhead for scale) [SE] Figure 8.6b Rip-up clasts of shale embedded in Gabriola Formation sandstone, Gabriola Island, B. The pieces of shale were eroded as the sandstone was deposited, so the shale is older than the sandstone.
The simplest and most intuitive way of dating geological features is to look at the relationships between them.
There are a few simple rules for doing this, some of which we’ve already looked at in Chapter 6.
The lower sandstone layer is disrupted by two faults, so we can infer that the faults are younger than that layer.