Unconformities are formed when rocks are eroded and removed; then covered by sedimentary strata.
In block diagrams, unconformities are often designated by a bold irregular line separating the erosion surface from the overlying sedimentary rock.
Three types of unconformities are pictured.
Disconformity:  sedimentary rock overlies undeformed sedimentary rock
Angular unconformity:  sedimentary rock overlies tilted, folded, or uplifted layers. 
Nonconformity: sedimentary rock overlies crystalline igneous or metamorphic rock that has been eroded.