An unconformity is an erosional surface between strata. Since they usually form when the older (lower) layers are exposed and eroded, unconformities can tell us about the history of the rock, but sadly, they can also erase millions of years of evidence. There are three main types.
- Disconformity: between parallel layers of sedimentary rocks
- Nonconformity: between sedimentary rocks and underlying older metamorphic/igneous rocks
- Angular uncomformity: between parallel layers of sedimentary rock and underlying tilted layers
It's easy to remember what angular unconformity stands for (angular = titled), but keeping disconformities and noncomformities apart can be harder. One trick is that disconformity is the only term with an "s", and "sedimentary" starts with "s"; therefore, a disconformity is an unconformity between parallel sedimentary layers.