<descriptor thcode="2"><thesaurus thcode="2" name="USGS Thesaurus" scope="Topics and methods of scientific study carried out by USGS, with product types, scientific disciplines, geologic time, and types of institutional structure and activities.  Broad and shallow, used to help people find scientific information." creator="U.S. Geological Survey" rights="Public domain" edition="2.0" date="2020-01-09" codetype="number" prefix="topic" uri="https://www2.usgs.gov/science/USGSThesaurus" tblname="term" nonpref="nonpref" relterm="relterm" root_code="1" contact="pschweitzer"><category><term thcode="25" code="29" name="distinct labels" parent="26" scope="Preferred labels (descriptor texts) are distinct within the thesaurus. A concept can be specified either by its unique identifier (code) or by the text of its descriptor."/><term thcode="25" code="16" name="hierarchical" parent="11" scope="Substantive broader-narrower relationships exist among the terms."/><term thcode="25" code="13" name="transitive" parent="12" scope="Narrower terms have an &quot;is a&quot; relationship with their broader terms, and generally can be described as part-of, type-of, or instance-of the broader term."/><term thcode="25" code="21" name="disciplines" parent="2" scope="Broad categories of human inquiry or activity, such as scientific disciplines."/><term thcode="25" code="20" name="methods" parent="2" scope="Actions taken to obtain or analyze information"/><term thcode="25" code="19" name="organizational functions" parent="2" scope="Things that organizations do, typically including various types of programs, projects, and services."/><term thcode="25" code="7" name="temporal" parent="2" scope="Terms indicate periods of time, such as chronostratigraphic units, named historical periods or intervals surrounding known historical events, or seasons."/><term thcode="25" code="5" name="theme" parent="2" scope="Terms indicate domains of human concern, such as scientific discipline, societal problems, or characteristics of earth, space, oceans, and living things."/><term thcode="25" code="29" name="distinct labels" parent="26" scope="Preferred labels (descriptor texts) are distinct within the thesaurus. A concept can be specified either by its unique identifier (code) or by the text of its descriptor."/><term thcode="25" code="16" name="hierarchical" parent="11" scope="Substantive broader-narrower relationships exist among the terms."/><term thcode="25" code="13" name="transitive" parent="12" scope="Narrower terms have an &quot;is a&quot; relationship with their broader terms, and generally can be described as part-of, type-of, or instance-of the broader term."/><term thcode="25" code="21" name="disciplines" parent="2" scope="Broad categories of human inquiry or activity, such as scientific disciplines."/><term thcode="25" code="20" name="methods" parent="2" scope="Actions taken to obtain or analyze information"/><term thcode="25" code="19" name="organizational functions" parent="2" scope="Things that organizations do, typically including various types of programs, projects, and services."/><term thcode="25" code="7" name="temporal" parent="2" scope="Terms indicate periods of time, such as chronostratigraphic units, named historical periods or intervals surrounding known historical events, or seasons."/><term thcode="25" code="5" name="theme" parent="2" scope="Terms indicate domains of human concern, such as scientific discipline, societal problems, or characteristics of earth, space, oceans, and living things."/></category><altlabel><altlabel>USGSThesaurus</altlabel><altlabel>USGSThesaurus</altlabel></altlabel></thesaurus><term thcode="2" code="1104" name="stratigraphy" parent="464" scope="Branch of geology concerned with the study of the formation, composition, ordering in time, and arrangement in space of stratified rocks."/><bt><term thcode="2" code="464" name="geology" parent="291" scope="Study of the planet earth, its composition, structure, physical and chemical processes, and history since its origin."/><term thcode="2" code="291" name="earth sciences" parent="1019" scope="Broad term for all science related to the study of the earth."/><term thcode="2" code="1019" name="sciences" parent="1" scope="Major educational fields, fields of study, and professional groupings within USGS."/><term thcode="2" code="1" name="Categories" scope="Thematic terms for categorizing information resources.  This thesaurus is intended to support finding and understanding scientific information."/></bt><nt><term thcode="2" code="115" name="biostratigraphy" parent="1104" scope="Branch of geology concerned with the separation and differentiation of rock units based on the fossils they contain."/><term thcode="2" code="668" name="lithostratigraphy" parent="1104" scope="Branch of stratigraphy dealing with the study of units of rock. Each unit is composed of a body of rock which is dominated by a certain lithology or similar color, mineralogic composition, and grain size."/></nt><rt><term thcode="2" code="84" name="bedrock geologic units" parent="1103" scope="Units of consolidated (solid) rock that underlie soils or other unconsolidated materials."/><term thcode="2" code="125" name="borehole logging" parent="375" scope="Method of recording the physical characteristics of the subsurface as a function of depth by observations during drilling of a well or borehole, studying the resulting core, or by lowering instruments into a drilled hole."/><term thcode="2" code="325" name="electrical resistivity logging" parent="125" scope="Recordings made to identify the composition of subterranean materials at different depths in a borehole by measuring their ability to resist the flow of an electric current."/><term thcode="2" code="424" name="gamma-ray logging" parent="125" scope="Recording of the measurements of natural gamma radiation emitted from the rocks in a borehole to study the subsurface structures."/><term thcode="2" code="438" name="geochronology" parent="464" scope="Study of the age of the earth by dating geological formations, rocks, and fossils."/><term thcode="2" code="452" name="geologic contacts" parent="1103" scope="Junctions of two different rock formations varying by type or age of rock, including faults, unconformities, and bedding planes."/><term thcode="2" code="455" name="geologic history" parent="287" scope="Record (and inferred reconstruction) of the origin and development of the earth since its formation."/><term thcode="2" code="1103" name="stratigraphic sections" parent="287" scope="Vertical arrangement in layers of rock units whose sequence is used to study the geological history of the strata."/><term thcode="2" code="1127" name="surficial geologic units" parent="1103" scope="Rock units on the earth's surface."/><term thcode="2" code="1198" name="unconformities" parent="452" scope="The contact between older rocks and younger sedimentary rocks where erosion had removed some of the older rocks before the younger rocks were deposited."/></rt></descriptor>