Cross cutting relationships used relative dating
Thus, we can deduce that the mudstone and shale are older than the rhyolite dike.
But, since the rhyolite dike does not cut across the shale, we know the shale is younger than the rhyolite dike.
Here is a lesson and accompanying resources for a year 11 GCSE Geology class.
The topic covered was stratigraphy and relative dating, so we covered many key geological concepts including the principle of unifominitarianism, cross-cutting relationships, original horrzontality, and the principle of superposition.
The Grand Canyon is particularly good for this because different sedimentary rocks have different colors.
For example, the mudstone, sandstone and shale are cut by the basalt dike, so we know that the mudstone, sandstone, and shale had to be present before the intrusion of the basalt dike.
Thus, we know that the dike is younger than the mudstone, sandstone, and shale.
Similarly, the rhyolite dike cuts only the mudstone and the sandstone, but does not cut across the shale.From the beginning of this course, we have stated that the Earth is about 4.6 billion years old.How do we know this and how do we know the ages of other events in Earth history?A more modern way of stating the same principle is that the laws of nature (as outlined by the laws of chemistry and physics) have operated in the same way since the beginning of time, and thus if we understand the physical and chemical principles by which nature operates, we can assume that nature operated the same way in the past.Principle of Superposition Because of Earth's gravity, deposition of sediment will occur depositing older layers first followed by successively younger layers.By carefully digging, we have found that each trash pit shows a sequence of layers.