Relative dating was the main technique used before the 20th century due to limited technology available for dating. This method of dating relies on making comparisons between sediment layers of rocks, this technique is used to make rough estimates.
Relative dating relies on two principles namely the principle of superposition and principle of correlation
This principle states that for particular strata (rock layers) that the oldest stratum is at the bottom of the rocks and the younger layers have formed on top.
This principle states that identification of index fossils (fossiled remains of organism used for dating) which will be found at various sedimentary depths can be used to identify rocks of similar ages. So if a particular fossil is found in two different levels of two different rocks than it is predicted that the two sections of rock existed at approximately the same geological time.
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