CLASSIFICATION OF ROCKSAND DESCRIPTION OF PHYSICALPROPERTIES OF ROCKIntroductionUniformity of definitions, descriptors, and identificationof rock units is important to maintain continuity ingeologic logs, drawings, and reports from a project withmultiple drilling sessions, different loggers and mappers.Also important is the recording of all significant observableparameters when logging or mapping. This chapterpresents a system for the identification and classificationof rocks and includes standard terminology anddescriptive criteria for physical properties of engineeringsignificance. The standards presented in this chaptermay be expanded or modified to fit project requirements.Rock ClassificationNumerous systems are in use for field and petrographicclassification of rocks. Many classifications requiredetailed petrographic laboratory tests and thin sections,while others require limited petrographic examinationand field tests. The Bureau of Reclamation (Reclamation)has adopted a classification system which is modifiedfrom R.B. Travis [1]. While not based entirely on fieldtests or field identification of minerals, many of theclassification categories are sufficiently broad that fieldidentification is possible. Even where differences in themineral constituents cannot be determined precisely inthe field, differences usually are not significant enough toaffect the engineering properties of the rock if classifiedsomewhat incorrectly by lithologic name. Detailedmineralogical identification and petrographicclassification can be performed on hand samples or coresamples submitted to a petrographic laboratory.

FIELD MANUALIf samples are submitted to a petrographic laboratory, thepetrographic classification generally will coincide with theclassification according to Travis. The petrographicigneous rock classifications are somewhat more preciseand include specialty rock types based upon mineral composition,texture, and occurrence. For example, alamprophyric dike composed of green hornblendephenocrysts or clinopyroxene in the groundmass could beclassified as spessartite, whereas a lamprophyrecontaining biotite with or without clinopyroxene could beclassified as a kersantite. Sedimentary rockclassifications generally include grain size, type of cementor matrix, mineral composition in order of increasingamounts greater than 15 percent, and the rock type, suchas medium-grained, calcite-cemented, feldspathicquartzosesandstone, and coarse- to fine-grained, lithicfeldspathic-quartzose gray-wacke with an argillaceousferruginous-calcareous matrix. Metamorphic rockclassifications include specific rock types based uponcrystal size, diagnostic accessory minerals, mineralogicalcomposition in increasing amounts greater than15 percent, and structure. Two examples of metamorphicrock descriptions are medium-grained, hornblende-biotiteschist, or fine- to medium-grained, garnetiferous,muscovite-chlorite-feldspar-quartz gneiss. The aboveclassification can be abbreviated by the deletion ofmineral names from the left to right as desired. Themineral type immediately preceding the rock name is themost diagnostic.The term "quartzite is restricted to a metamorphic rockonly. The sedimentary sandstone equivalent is termed a"quartz cemented quartzose sandstone."Samples submitted to a petrographic laboratory should berepresentative of the in-place rock unit. For example, ifa granitic gneiss is sampled but only the granite portionsubmitted, the rock will be petrographically classified as a granite since the gneissic portion cannot be observed orsubstantiated by the thin section and hand specimen.Petrographic classifications can be related to theengineering properties of rock units and are important.Geologic rock unit names should be simple, and generalrock names should be based on either field identification,existing literature, or detailed petrographic examination,as well as engineering properties. Overclassification isdistracting and unnecessary. For example, use "hornblendeschist" or "amphibolite" instead of "sericitechlorite-calcite-hornblende schist." The term "granite"may be used as the rock name and conveys more to thedesigner than the petrographically correct term"nepheline-syenite porphyry." Detailed mineralogicaldescriptions may be provided in reports when describingthe various rock units and may be required to correlatebetween observations, but mineralogical classificationsare not desirable as a rock unit name unless the mineralconstituents or fabric are significant to engineeringproperties.The classification for igneous, sedimentary, metamorphic,and pyroclastic rocks is shown on figures 4-1, 4-2, 4-3, and4-4, respectively. These figures are condensed andmodified slightly from Travis' classifications, but themore detailed original classifications of Travis areacceptable. Figure 4-5 or appropriate AmericanGeological Institute (AGI) data sheets are suggested foruse when estimating composition percentages inclassification.Description of RockAdequate descriptors, a uniform format, and standardterminology must be used for all geologic investigationsto properly describe rock foundation conditions. Theseparagraphs provide descriptors for those physical characteristicsof rock that are used in logs of exploration, innarratives of reports, and on preconstruction geologicmaps and cross sections, as well as construction or "asbuilt"drawings. The alphanumeric descriptors providedmay be used in data-field entries of computer generatedlogs. Chapter 5 establishes descriptors for the physicalcharacteristics of discontinuities in rock required forengineering geologic studies.All descriptors should be defined and included in alegend when submitting data for design and/or records ofconstruction. An example of a legend and explanation isfigure 4-6, Reclamation standard drawing 40-D-6493,may be used for geologic reports and specifications wherethe standard descriptors and terminology established forrock are used during data collection.

Format for Descriptions of RockEngineering geology rock descriptions should include generalizedlithologic and physical characteristics usingqualitative and quantitative descriptors. A generalformat for describing rock in exploration logs and legendson general note drawings is: Rock unit (member or formation) name Lithology with lithologic descriptorscomposition (mineralogy)grain/particle sizetexturecolor Bedding/foliation/flow texture Weathering Hardness/strength Contacts Discontinuities (includes fracture indexes) Permeability data (as available from testing) Moisture conditionsExample descriptions are presented in a later section.

Rock Unit Names and IdentificationRock unit names not only are required for identificationpurposes but may also provide indicators of depositionalenvironment and geologic history, geotechnical characteristics,and correlations with other areas. A simpledescriptive name and map symbol should be assigned toprovide other users with possible engineeringcharacteristics of the rock type. The rock unit names maybe stratigraphic, lithologic, genetic, or a combination ofthese, such as Navajo Sandstone (Jn), Tertiary shale(Tsh), Jurassic chlorite schist (Jcs), Precambrian granite(Pcgr), or metasediments (ms). Bedrock units of similarphysical properties should be delineated and identified asto their engineering significance as early as possibleduring each geologic study. Planning study maps andother large-scale drawings may require geologicformations or groups of engineering geologic units withdescriptions of their engineering significance inaccompanying discussions.Units should be differentiated by engineering propertiesand not necessarily formal stratigraphic units wheredifferences are significant. Although stratigraphicnames are not required, units should be correlated tostratigraphic names in the data report or by anillustration, such as a stratigraphic column.Stratigraphic names and ages (formation, member) maybe used as the rock unit name.For engineering studies, each particular stratigraphicunit may require further subdivisions to identifyengineering parameters. Examples of importantengineering properties are: Susceptibility to weathering or presence ofalteration Dominant discontinuity characteristics Hardness and/or strength Deformability Deletereous minerals or beds (such as swellingsusceptibility, sulfates, or clays)For example, a Tertiary shale unit, Tsh, may bedifferentiated as Tsh1 or Tsh2 if unit 2 contains bentoniteinterbeds and unit 1 does not, and Tshc could be used asa unit name for the bentonite beds. A chlorite schist unit,Cs, may be differentiated as CsA or CsB where unit Acontains higher percentages of chlorite or talc and issignificantly softer (different deformation properties) thanunit B. A metasediment unit, ms, may be furtherdifferentiated on more detailed maps and logs as mssh(shale) or msls (limestone). All differentiated units shouldbe assigned distinctive map symbols and should bedescribed on the General Geologic Legend, Explanation,and Notes drawings.

BIBLIOGRAPHY[1] Travis, Russell B., "Classification of Rocks," v. 50,No. 1, Quarterly of the Colorado School of Mines,Golden, CO, January 1955.[2] Fisher, R.V., "Rocks Composed of Volcanic Fragments:Earth Science Review, v. I, pp. 287-298, 1966.[3] Williams, H., and McBirney, A., Volcanology,published by Freeman, Cooper and Company,San Francisco, CA. 391 pp., 1979.[4] Compton, Robert R., Geology in the Field, publishedby John Wiley & Sons, Inc., New York, NY, 1985.[5] Geological Society of America Rock Color Chart,8th printing, 1995.