The surface that separates two rock units, if the rock layers on either side (top and bottom) are parallel, it is a parallel unconformity; if they lie at an angle to each other, it is an angular unconformity. For example, a layer of sandstone is lying on a layer of limestone. Where the two dissimilar formations touch or meet, this surface is an unconformity; the upper layer does not conform to the lower layer or vice versa.
