There are three kinds of unconformities: disconformities, nonconformities, and angular unconformities. Disconformities. Disconformities (Figure 1 ) are usually erosional contacts that are parallel to the bedding planes of the upper and lower rock units.

A fossil is the preserved remains or traces of a dead organism. The process by which a fossil is formed is called fossilisation. After an animal dies, the soft parts of its body decompose leaving the hard parts, like the skeleton, behind. This becomes buried by small particles of rock called sediment.

1 : nonconformity. 2 : a break in a sequence of sedimentary rocks all of which have approximately the same dip.

The Law of Superposition, which states that older layers will be deeper in a site than more recent layers, was the summary outcome of ‘relative dating’ as observed in geology from the 17th century to the early 20th century.

Law of superposition, a major principle of stratigraphy stating that within a sequence of layers of sedimentary rock, the oldest layer is at the base and that the layers are progressively younger with ascending order in the sequence.

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A fault is a break in Earth’s crust. A fault is always younger than the rock it cuts through. The surface where new rock layers meet a much older rock surface beneath them is called an unconformity. An unconformity is a gap in the geologic record.

The principle of cross-cutting relationships states that a fault or intrusion is younger than the rocks that it cuts. The fault labeled ‘E’ cuts through all three sedimentary rock layers (A, B, and C) and also cuts through the intrusion (D). So the fault must be the youngest formation that is seen.

To determine the relative age of different rocks, geologists start with the assumption that unless something has happened, in a sequence of sedimentary rock layers, the newer rock layers will be on top of older ones. This is called the Rule of Superposition. …

The principle of cross-cutting relationships states that a fault or intrusion is younger than the rocks that it cuts through. The fault labeled ‘E’ cuts through all three sedimentary rock layers (A, B, and C) and also cuts through the intrusion (D). So the fault must be the youngest formation that is seen.

Fault H is older than sedimentary layers because the fault has been there before the layers because the layers are the youngest layers.

Beneath the surface, magma may push into bodies of rock. There, the magma cools and hardens into a mass of igneous rock called an intrusion. An intrusion is always younger than the rock layers around and beneath it. A fault is always younger than the rock it cuts through.

However, many layered rocks are no longer horizontal. Because of the Law of Original Horizontality, we know that sedimentary rocks that are not horizontal either were formed in special ways or, more often, were moved from their horizontal position by later events, such as tilting during episodes of mountain building.

Angular unconformity also causes geologic tilting. Angular unconformity occurs when parallel strata of sedimentary rocks are deposited on layers that are tilted, perhaps as a result of erosion.