Relative dating of geological features
But why does this adjacent crater have so many more boulders?
LROC WAC monochrome mosaic showing the wrinkle ridge in Mare Imbrium.Today's Featured Image provides an example of a clear age relation, where an impact crater is cross-cut and deformed by a wrinkle ridge.The deformation of this crater took place in the north-south direction, and distorts the crater from a circular shape to ellipse.At some point after the impact event, the wrinkle ridge deformed the crater.However, much time has passed since the impact event, because the crater has no high-reflectance ejecta rays, its walls are smooth, and there are only a few boulders.These observations help us construct a plausible geologic story for this small area.
First, the mare basalts erupted and cooled, then the impact crater formed.
It may surprise you to learn that geologists were able to determine much of the history of the Earth and its life without knowing anything about the actual ages of the rocks that they studied.
Through use of techniques (which were developed during the 20th century; see Section 2.2), they were able to later assign dates in years before the preset to important events in Earth's history.
The image below shows a sequence of Devonian-aged (~380 Ma) rocks exposed at the magnificent waterfall at Taughannock Falls State Park in central New York.
The rocks near the bottom of the waterfall were deposited first and the rocks above are subsequently younger and younger. This work is licensed under a Creative Commons Attribution-Share Alike 4.0 International License.
Taughannock Falls near Trumansburg, New York, illustrating the Principle of Superposition. Superposition is observed not only in rocks, but also in our daily lives. The trash at the bottom was thrown out earlier than the trash that lies above it; the trash at the bottom is therefore older (and likely smellier! Or, think about a stack of old magazines or newspapers that might be sitting in your home or garage: most likely, the newspapers at the bottom of the pile have dates on them that are older than the newspapers at the top of the pile.