In Geology an unconformity is a surface separating two rock types of different ages from one another. An unconformity represents a period of erosion or non-deposition in the sedimentary record, i.e. a gap or hiatus where we have no record of what happened.
One such unconformity is Hutton’s Unconformity. The confusing part is that the name Hutton’s Unconformity is the name given to several unconformities identified by the famous 18th century Scottish geologist James Hutton. On a recent fieldtrip to Scotland we visited Hutton’s Unconformity at Siccar Point, but other unconformities identified by James Hutton can be found on Arran and near Jedburgh.
When sediments (fragments of previous rocks) are deposited (settle before becoming a rock) they form layers of progressively younger material burying the older material. If a sedimentary rock is the correct way up the further up the rock you go the younger (closest to present day) it is.
At Siccar point we can see two distinct units rock, with the layers of rock (bedding or strata) in the upper and lower units being at different orientations from one another. This means that before the second (upper) unit was deposited the lower (older) unit was tilted and eroded, producing a gap in the geological record.
Unconformities are crucial to our current understanding of geology. They paved the way for us to consider deep time, that is essentially an appreciation of the fact it has taken a very long, yet quantifiable length of time for the rocks we observe around us to have reached their current situation. For the rocks here at Siccar point that means; deposition of the older sediments, tilting of the older sediments, erosion, then the deposition of the younger sediments on top. This order of processes cannot possibly happen quickly!
Unconformities have also been used as evidence that the mechanisms governing the production of these rocks (and the universe) have, and always will operate in an assumption known as uniformitarianism. Uniformitarianism is a counter (and more accepted view) than the opposing catastrophism, which implies that the Earth was created in a series of sudden short lived events.