Layers of sediments are generally deposited on top of each other. But sometimes sequences of different age are separated by an erosion surface. The difference in age between the older and younger rocks, i.e. the time gap, is called an hiatus.
The foto below shows an angular unconformity at Vallys Vale, eastern Mendips, southwest England (Foto by Ian Somerville, UCD). Jurassic Oolites (ca. 170 Ma) with horizontal bedding overlie tilted Lower Carboniferous limestones (ca. 345 Ma). The time gap (hiatus) between sedimantation of the two sequences is therefore ca. 175 million years!

angular unconformity

The paper model consists of two parts, representing the strata below and above the unconformity. The model illustrates how unconformities can be detected both in cross-section and map view. Note that the angular relationships between the strata below and above the unconformity depend on the orientation of the cross-section. One section suggests a low angle unconformity whereas another section, at right angles, suggests a high angle unconformity. Using a stereonet try to work out the orientation of bedding in the older rocks at the time they were eroded. Hint: The younger rocks have bedding parallel to the unconformity, suggesting that the unconformity was originally horizontal; this assumption is not necessarily valid but is fine for the purposes of this exercise.

Unconformity model