Geology is defined as the scientific study of the history of the earth, especially as indicated in its rocks. Rocks are classified according to their mode of formation, "igneous" from a molten mass, "sedimentary" from deposition in a fluid, commonly water, "metamorphic" from chemocrystalline changes due to heat and pressure, and "pyroclastic," literally "fire broken," but much better described as "volcanoclastic.'' However, the mode of rock formation is not the same as the geologic formation. The geologic formation is defined as any rock mass that is mappable. If there is one or more common characteristics of a rock mass exposed on the surface, it can be mapped. As an extreme example, a single rock color could be used to define a formation; however, a variety of lithologic characteristics commonly are used not only because they serve to distinguish one formation from another but also they infer their genesis which helps to answer the geologist's basic question: How did things come to be as they are?

It is useful to give names to geologic formations of any relatively large area. In some cases, a named formation may cover hundreds of square miles, but depending on the area of interest, it may be convenient to name a formation having a much smaller areal extent. Generally, such names are taken from geographic features, combined where appropriate, with the rock type, e.g. "Monterey Shale." Furthermore, within a formation there may be one or more other mappable units commonly referred to as a "members," and finally, there can be members of lesser extent, i.e., members within members, to which the term "unit" may be applied.

A somewhat peculiar habit has developed among geologists in the case of the landslide of which there have been many in Malibu. A landslide is actually a process. It commonly is defined as the "relatively rapid, downward and outward, movement of a mass of earth material in response to gravity." The mass that has slid is landslide debris, but many geologists incorrectly refer to the debris as a landslide. Since landslide debris is almost always mappable, it therefore can be regarded as a geologic formation and one to which a name commonly is applied. In this regard, a certain degree of prudence is to be considered. It is much better to name a landslide in the same way as are most formations, i.e, according to some geographic feature. For example, it is much better to say "the Oak Street landslide," rather than "the John Smith landslide" as Mr. Smith would be quick to tell you.

Matters are complicated to some extent because different geologists, working in different areas have given different names to what later is determined to be the same formation. Furthermore, detailed work in an area with an earlier named formation may show that within it there are units deserving separate formation status. This may lead to considering the original formation as a "group" of formations. This scheme of naming various earth units is referred to as "geologic nomenclature." In the Santa Monica Mountains, an earlier nomenclature devised largely by U.S. Geologists Survey geologists in late 19th and early 20th centuries has been revised to some extent since 1979 by other U.S. Geological Survey geologists. Depending on the nomenclature used, there are some eight to fifteen geologic formations recognized in Malibu some of which occur in other areas as well. The manner in which these formations affect living conditions is the reason that those who are interested in Malibu should understand the concept of the geologic formation.

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