## Dr. Michael Wilson

Geology Department, Douglas College

### Relative and Absolute Dating

Relative dating would be the placement of something in a sequence. So if we establish a sequence for an area we simply need to know what came first, then what came next, etc. For example, in a sequence of stratified deposits, laid down by a river or deposited by wind, we are talking about superposition as the basis for that sequence. Superposition means that the oldest deposits would be at the bottom and the youngest deposits would be on the top, in kind of layer cake fashion. So that principal has served us very well. If we have no means by which to absolutely date materials, we can still work out a sequence of events – that can be extremely important. So relative dating was really the only mechanism that people had available for dating archaeological and paleontological materials until the 20th century.

There were a few absolute dating methods, and by absolute, I mean dating with respect to a measured time series. In other words, being able to place something in terms of a calendar– dating in terms of years. There were a few techniques that could be used in certain situations – counting of annual cycles in glacial lakes – we call these cycles “varves”. Counting those annual cycles of sediment deposition could actually allow us to determine the age of these lake deposits if they continued up to the present day. Or the use of tree ring dating – counting the rings in trees if we have logs preserved in a fairly recent site and could relate it to some known historic trees. Those were absolute dating techniques. But for the most part, until the 20th century, relative dating was the basis for archaeological, paleontological and geological dating.