1. Sedimentary rocks generally occur in layers that reflect the pattern of sedimentation of the original sediments. This layering of sedimentary rocks is referred to as stratification.

1) Figure 9.3: Nicholas Steno was an Italian scientist who recognized that many rocks units were deposited in horizontal layers, a phenomenon called stratification. In 1669, Steno stated three basic principles for chronological analysis of the rock record which have proven to be extremely useful in the study of earth history.

(a) Principle of superposition: In a succession of undeformed strata, the oldest stratum lies on the bottom with successively younger ones above.

(b) Principle of original horizontality: Most strata were initially deposited horizontally. Strata that are inclined or folded must have suffered disturbance subsequent to deposition.

(c) Principle of original lateral continuity: Strata originally extended in all directions until they thinned to zero or terminated against the edges of the basin of deposition.

1. Figure 9.4: Life evolved and changed throughout earth history.

2. Rocks of different ages therefore contain different types or assemblages of fossils.

3. Rocks of a certain age contain one set of fossils that differ from fossils in rocks of different ages.

1. Figure 9.5: Rock units are not continuously exposed. Rock exposures often are separated by great distances. Between exposures, rocks were either buried or have been eroded away.

2. Correlation involves matching a particular rock unit in one exposure with its counterpart at a different locality using criteria such as color, mineral composition, characteristic fossils etc. By correlateing various rock exposures separated by great distances, geologic maps can be constructed and the original geographical extent of the rocks can be estimated.

3. Figure 9.5: Studies of fossils preserved within the rock record indicate that rocks of different ages contain different types or assemblages of fossils. This is because life evolved and changed throughout earth history. As a result, rocks of a certain age contain one set of fossils that differ from fossils in rocks of different ages. The study of the history of ancient life from the fossil record is called paleontology.

4. The principle of fossil correlation states that like assemblages of fossils are the same age and therefore strata containing these particular fossils are also the same age. Fossils are therefore very useful in correlating, or matching up rock sequences in exposures separated by great distances.

5. Rock units can also be correlated using color, mineral composition, chemical composition, sedimentary structures and other criteria.

1. Stratigraphy is a sub-science of geology that attempts to organize the rock record into some manageable classification scheme. The science of stratigraphy has its roots in the late 1700s and early 1800s as a consequence of the extensive geologic mapping that took place in Great Britian and western Europe during that time.

2. Figure 9.5 & Box 9.1: The most basic local unit of stratigraphy is the formation. A particular rock unit is grouped into a formation based on a distinctive appearance and/or other characteristics. (a) Sometimes a formation can be defined simply on the basis of rock type. In other cases, however, (b) adjacent units may be of the same lithology and differ only in color such as green shale overlying brown shale, or (c) a rock sequence may be interstratified with two or more lithologies. In these latter two cases, some arbitrary division must be chosen.

3. Formations are typically named after the locality where the rock sequence is best exposed and/or serves as the basis for the classification.

4. The thinnest rock unit observable within a formation is called the stratum.

5. On a large scale, several formations can be combined into one group.

6. Rock units can also be divided on the basis of fossils into zones that are named after key index fossils. Zone boundaries often do not match formation boundaries distinguished on the basis of lithology.

1. Unconformities represent erosional surfaces or intervals of missing strata. Like formations, unconformities can be traced and mapped over great distances. Unconformities, like some formations, can vary in age from place to place. The amount of missing rock section represented by an unconformity can also vary from place to place.

2. Figures 9.6 & 9.8: In some cases, unconformities can represent profound crustal upheaval and deep erosion. In other cases, the unconformity may represent very little erosion or simply be a surface of nondeposition without erosion.

3. There are several types of unconformities:

(a) A disconformity is an erosional surface where there is no discordance between strata below and above the surface of discontinuity.

(b) An angular unconformity (Figs 9.7 & 9.8) is an erosional surface where there is an angular discordance between the older and younger strata.

(c) A nonconformity occurs where the underlying rocks are igneous or metamorphic.

4. A single unconformity can vary in type from place to place and eventually be traced to a point where the erosional surface disappears. At this point, the underlying strata becomes conformable with the overlying unit and all rocks are represented.

1. Other disturbances can also provide clues for dating sequences of rocks.

A) Figure 9.9: Dikes and Sills: Rocks that have been intruded by dikes or sills must be older than the intrusion.

B) Figure 9.9: Major Displacements in Rock Sequences: Faults are planes of detachment within rock bodies. Each detached unit can be referred to as a block. Rocks in one block that have been detached (offset) from their counterparts in another block must be older than the fault itself.

C) Figure 9.9: Folds: Rocks that have been folded must be older than the folding event.

1. Fragments of a rock unit enclosed in another rock unit.

2. The rock mass that supplied the inclusion material must be older than the rock containing the inclusion.