Obviously, the sedimentary rocks had to be there prior to emplacement of the igneous rocks, and thus they are older than the igneous rocks. If long-term cratering rates are known to enough precision, crude absolute dates can be applied based on craters alone; however, cratering rates outside the Earth-Moon system are poorly known. Underpinning this are four key concepts:. As a result, xenoliths are older than the rock which contains them. The diagram right shows two distinct faunal assemblages. Underpinning this are four key concepts:
Deep time Geological history of Earth Geological time units.
However, the layer of that material will become thinner as the amount of material lessens away from the source. The image at left illustrates faunal succession. In other words, as sediment fills a depositional basins we would expect the upper most surface of the sediment to be parallel to the horizon. As it is free swimming it could have travelled a considerable distance. Principal of Original Horizontality i. Geologists still use the following principles today as a means to provide information about geologic history and the timing of geologic events.