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Unconformities - ancient surfaces of erosion

An unconformity is an ancient erosion surface - a place where younger stratified rocks have been deposited on older, typically deformed strata.

Traditionally, unconformities are classified into three categories.

If the strata above and below the unconformity surface are parallel, the unconformity is a disconformity.
If the rocks below the unconformity were tilted before the deposition of the upper unit, then the surface is an angular unconformity
If the rocks below the unconformity are not clearly stratified (e.g. plutonic igneous rocks) then the unconformity is a nonconformity.

Disconformity: Middle Ordovician Table Point Formation limestone (grey) on Early Ordovician Aguathuna Formation dolostone (yellow-grey), Aguathuna Quarry, Port au Port Peninsula, Newfoundland, Canada.

Angular Unconformity: Triassic Wolfville Formation on Carboniferous Horton Group, Whale Cove, Nova Scotia, Canada.

nonconformity Neoproterozoic Torridon Group on Paleoproterozoic Lewisian gneiss, northwest Scotland, UK.