The convention in geology is to number the layers (beds) within a sequence such that the oldest layer has the lowest number. Such sequences (from oldest to youngest) might be 1, 2, 3, 4….etc. or 23, 24, 25, 25, 25…. etc.

In the illustration, layer 1 was deposited at time 1. At time 2, layer 2 was deposited on top of layer 1. At time 3, layer 3 was deposited on top of layer 3. Etc., etc.

1. During a certain period of time, while layers of sediment were being deposited elsewhere, no layers were deposited at the location in question.
   Or
2. Layers were deposited at the location in question, but were subsequently removed by erosion.

• At location C, layers 1 through 5 were deposited and remained intact. The rock record is complete.
• At location A, layers 1 and 2 were deposited. However, during times 3 and 4, no layers were deposited. During time 5, deposition resumed, and layer 5 was deposited.
• At location B, layers 1 through 3 were deposited. During time 4, all of layer 3 plus the upper part of layer 2 were removed by erosion. During time 5, deposition resumed, with layer 5 being deposited on top of what remained of layer 2.

Gaps in the rock record, due to non-deposition and/or erosion, are called unconformities. Unconformities caused by erosion are commonly represented diagrammatically by an irregular or jagged line, such as is seen between layers 2 and 5 at location B.

In metamorphic rocks, layering may develop in response to application of pressure. In that case, the layers may all form at the same time. The position of a layer within the series, above or below another layer, will not be indicative of whether it is younger or older.

An important question, therefore, is how may cross-section C (in which the sill is younger than layer 30) be distinguished from cross-section D (in which the sill is older than layer 30)?

Finding an answer to that question will be discussed in subsequent sections.

At location A, three layers are present. They have not been deformed and remain as originally deposited. The layers are covered except for the area within the circle. Looking at the exposed layers and applying the Law of Superposition, an observer concludes correctly that the bottommost layer (dark brown) is oldest and the topmost layer (orange-tan) is youngest.

At location B, the layers are slightly folded. A second observer, who has not been to location A, sees slightly inclined layers and concludes correctly that the layers have been somewhat deformed, but that the topmost layer is the youngest and the bottommost the oldest.

At location C, the layers have been tightly folded. In the exposed circled area, the layers are vertical. A third observer, who has not been to locations A or B, sees the vertical layers and cannot decide which layer was originally 'topmost' and which 'bottommost' and draws no conclusion about their relative ages.

At location D the layers have undergone extreme deformation. The layers within the circled area have actually been inverted. What now appears to be the 'topmost' layer was originally the 'bottommost' (compare with the order of the layers in Diagram A). A fourth observer, who has not been to locations A, B or C, sees the almost horizontal layers and assumes (incorrectly) that the layers have not been significantly deformed. Applying the Law of Superposition to determine the relative ages of the layers, the observer gets the relative ages of the layers reversed.

1. graded bedding: when a mass of different sized grains settle out through water to form a layer of sediment, course grains predominate at the bottom of the layer, fine grains at the top.
2. wave ripple marks: when waves disturb sediment on the floor of the ocean or a lake, ripples form. The points of the ripples point upward.
3. raindrop impressions: when rain drops plow into soft mud, craters form. The crater basins are convex down; the crater rims point up.
4. the orientation of included fossils: when empty, disaggregated clam shells are disturbed by waves on the ocean floor, most of the shells end up with the outer (convex) side of the shell pointing upoward. The branches of tree roots point downward.
5. vesicles in lava flows: vesicles are concentrated near the upper surface of a lava flow.

Diagram A illustrates an extensive outcrop of horizontal layers exposed over a great distance. The layers have a high probability of being 'right-side-up'.

Diagram B illustrates several separated local outcrops in which horizontal layers are exposed. The layers in the separate outcrops 'line up' with one another. The geologist assumes (dashed lines) that if the grass and soil were removed, the layers would be continuous over the whole area. The layers have a high probability of being 'right-side-up'.

Diagram C illustrates a single local outcrop of horizontal layers. Because completely inverted layers are rare (layers turned right over to become horizontal again), the geologist assumes, in the absence of contrary evidence, that the layers are probably 'right-side-up'.

The modern interpretation of fossils is that they actually are remains or artifacts of once living organisms. Normally, after living organisms die, their remains are quickly scattered and decayed and the record of their existence is rapidly obliterated. On rare occasions, quick burial of the remains by mud, sand or volcanic ash prevents their destruction and they become preserved as the loose material in which they are embedded is lithified. The preservation of soft parts of organisms is extremely rare. Preserved hard parts are commonly mineralized (turned into rocky substances).

Snails

Fern leaves.

• A sequence of rock layers numbered 52 to 63 exposed at location 'X'. One of the rock layers, (55), exhibits graded bedding, indicating the layers are 'right-side-up'. Hence, layer 52 is oldest, layer 63 is youngest. Each layer formed during a certain period of time and represents what happened at location 'X' during that time.
• A series of colored dots that represent the levels within the rocks where specimens of fossil species A, B, C, and D have been found at location 'X'. Each level represents a moment in time.
• A series of colored double-headed arrows indicating the range of time spanned between the lowest and highest levels of the occurrence of each fossil species at location 'X'.
• A series of black double-headed arrows indicating the range of time spanned between the lowest and highest levels of the occurrence of each fossil species found in rocks throughout the world.

The time interval between the first and last appearance anywhere in the world of a fossil species is known as its 'geologic range'. With continued investigation, The geologic ranges of individual species are subject to revision as investigation of rocks continues. Newly discovered occurrences may place the introduction and extinction of species respectively earlier and later in time. As the geologic ranges of species are adjusted, the geologic ranges of fossil assemblages are also revised.

Correlating rocks in some cases is simple. In others, it can be a complex process involving many observations, hypotheses and tests of hypotheses. As will be seen, fossils frequently play a vital role in correlation.

First, consider a relatively simple case. A grassy slope displays three outcrops of horizontally layered rocks (diagram A). The geologist notes that the sequence and characteristics (thickness, color, texture, mineralogy) of the layers in the three outcrops are the same.

This argument is based upon two important assumptions.

• It is assumed that it is highly probable that the layers observed in each outcrop continue laterally underneath the grass beyond each outcrop. This assumption is known as the Law of Lateral Continuity: Most sediments are laid down as layers on flat surfaces and have considerable extent in all directions compared to the thickness of the layer. For example, pulses of sediment flushed into in a lake may come to rest as inch-thick layers deposited over a considerable portion of the lake floor. Thus it is reasonable to assume that the layers seen in the separated outcrops are actually joined.

• It is assumed that it is highly probable that each layer has the same age throughout its length and breadth. In the lake example, all parts of each pulse of sediment brought to the lake settle to the lake floor at roughly the same time.

Consider the diagram shown below. Recall that the black double-headed arrows represent the worldwide geologic ranges of fossil species A, B, C and D. That is, no specimens of these fossil species have been found anywhere in the world in rocks older or younger than the indicated ranges. In constructing the ranges of the fossils, hundreds of localities (including location 'X') have been examined and the accuracy of the geologic ranges is considered established: after examination of the first few dozen localities, no adjustments had to be made to the ranges.

The following conclusions may be drawn:

• The rock colored blue at location 'X' formed before fossil B disappeared from the rock record and after fossil C appeared in the fossil record.
• Rocks at location 'Z' have not been examined before. The rock colored blue at location 'Z' has the same fossils as those in the 'blue' rock at location 'X'. Because the geologic ranges of fossils B and C are considered well-established, it is judged highly likely that this rock formed within the same time interval as that at 'X': between 'p' and 'q'.
• Although both 'blue' rocks formed within the same time interval - between 'p' and 'q' - they did not necessarily form at precisely the same time. It may be that the rock at 'X' formed towards the middle of the time interval, whereas the rock at 'Z' formed towards the end of the time interval. However, if the time interval is considered short, the 'blue' rocks at 'X' and 'Z' may be said to correlate. How the length of the time interval is measured will be considered when 'absolute age' determination is discussed.
• The rock colored pink at location 'X' formed before fossil A disappeared from the rock record and after fossil B appeared in the fossil record, within the time interval between 'm' and 'n'. The same is judged to be true for the rock colored pink at locations 'Y' and 'Z'. Thus, all three 'pink' rocks may be said to 'correlate' with each other.
• The 'blue' rocks are younger than the 'pink' rocks.

• It is worth emphasizing that rocks of the same age need not be similar. In the above example, the lake mud and the landslide debris form deposits whose characteristics are very different. The mud is fine grained and forms a thin but extensive layer. The landslide deposit is restricted to a small area, and forms an irregular, relatively thick layer composed of an assortment of large to small fragments. When the deposits are lithified, the differences in their character are preserved in the rocks they form.

• Conversely, rocks that are highly similar in character (aside from fossil content), may have formed at quite different times in earth history.

• Despite these complications, geologists over the last 200 years have worked out in great detail the sequence, distribution and equivalency of fossils in rocks all over the world. From this knowledge, relative ages of geographically widely separated rocks have been determined. The work is ongoing and continues today.

• Where a rock is cut by a fracture, the fracture is younger than the rock. After all, how could the rock be fractured if it wasn't already there?
• Where a body of igneous rock 'A' intrudes some other rock 'B', rock 'B' is older than rock 'A'.

  However, caution has to be employed when interpreting cross-cutting rock bodies.

  • Whether or not 'intrusion' is the cause of the cross-cutting relationship must be ascertained.

  • If two 'intrusive' bodies intersect, coming to a conclusion as to which formed first depends on assumptions as to probability.
• Where a rock is cut by an erosion surface, the erosion surface is younger than the rock it cuts.

The law has to be applied with care, however, because some rocks contain rocky objects that develop after the rock formed. Perfectly formed crystals and lumpy objects known as concretions can be deposited from groundwater as it flows through sedimentary rocks. Skill is needed to recognize such objects and to distinguish them from ordinary inclusions.