Rules of relative dating
(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.[SE] The principle of cross-cutting relationships states that any geological feature that cuts across, or disrupts another feature must be younger than the feature that is disrupted.A chance encounter between determined fishermen and a great white shark off the Tuscan coast in 1666 sparked a chain of events that would help change humans views of fossils and Earth’s geologic past (Cutler 2003, pp. Nicolas Steno (1638-1686) dissected the head of this shark and realized fossil tongue stones believed to be petrified snake or dragon tongues were actually fossil shark teeth (Prothero 1998, p. One problem still existed, how do fossils become embedded in solid rock?Steno recognized that fossils represent organisms that became buried in sediment, which later turned into rock.An example of this is given in Figure 8.7, which shows three different sedimentary layers.The lower sandstone layer is disrupted by two faults, so we can infer that the faults are younger than that layer.This law was independently discovered by William Smith (1769-1839), a British engineer, while working on excavations for canals in England (Winchester, 2002 p.
Playfair later commented that, "the mind seemed to grow giddy by looking so far into the abyss of time." Mc Phee (1998) points out that Hutton removed humans from a specious place in time just as Copernicus had removed humans from a specious position in the universe (p. Hutton gives us three more laws to consider when seeking relative dates for rock layers, one of which, the law of inclusions was described earlier.states any feature that cuts across a rock or sediment must be younger than the rock or sediment through which it cuts.
Steno formalized the laws of superposition, original horizontality, original continuity and inclusions in his publication entitled states that any inclusion is older than the rock that contains it.
Steno's idea that fossils are older than the rock in which they are found hints at this principle, but Hutton is most often given credit for this principle.states that fossil organisms succeed one another in a definite, irreversible, and determinable order.
The image is about 7 m across.) [SE photo] An unconformity represents an interruption in the process of deposition of sedimentary rocks.
Recognizing unconformities is important for understanding time relationships in sedimentary sequences.There are a few simple rules for doing this, some of which we’ve already looked at in Chapter 6.