Relative dating methods use geological principles to place events in
Events can be ordered using the various methods discussed even when no absolute
ages can be measured. Where there is a break in sedimentation, a period of
erosion or an episode of deformation, the rock layers record the break as a
surface called an unconformity. Unconformities range from minor erosional
breaks to strong angular discordances in bedding. An unconformity indicates a
period where no rock record is accumulated. They are time-breaks of
indeterminate length. The last two diagrams illustrate various kinds of
- The principle of superposition states that older beds are covered
by younger beds so in a sedimentary sequence the youngest unit is at the top.
- The principle of fossil succession states that organisms evolve
through time so that particular forms can be used as age markers wherever they
- Cross-cutting relationships Cross-cutting rocks such as igneous
intrusion are younger than the rocks they cut.
- Inclusions Any included pebbles and fragments
must be older than the host rock containing them.
- Deformation Any rocks effected by a deformation event
(folding or tilting) must pre-date the deformation episode.
(c) Department of Civil and Geological Engineering, University of Saskatchewan,
57 Campus Drive, Saskatoon, SK, Canada, S7N 5A9