The consulting relationship between geologists and archaeologists/historians is designed to achieve a fuller understanding of a site's occupational and socioeconomic story, ideally as excavation is taking place. Another term for archaeological geology is geoarchaeology, the activity in which a geological scientist engages in historical/archaeological analyses and diagnoses from an empirical base.

The locus, a fundamental unit of archaeological stratigraphy, has discrete geological analogues in the bed, tongue, or layer units of a sequence of rock strata in any exposure/outcrop of unconsolidated sediments anywhere on earth. Sedimentary analytic techniques can be applied to a locus to determine its geological makeup: texture, fabric, particle size, micro/macrobedding, mineral/lithic, and material science diagnosis. (the identification of any artifact and its chemical composition and its function in human applications). [See Locus.] Studies propose multiple working hypotheses, which set forth probable agencies/events of origin and form the operational archaeological geological research basis.

Geological investigations in historical environmental remains may draw from various subsciences to address the action and interaction of human and natural depositional and erosional activities in, about, and regionally near a site: mineralogy (elements, salts, compounds), sedimentology (origin, movement, accumulation of particles), stratigraphy (forms of layering), structure (spacial geometry of any lithic body), petrology (igneous, sedimentary, and metamorphic rock origin and description), geomorphology (genesis and shape of the earth's surface), aerial photography (database for mapping), geohydrology (presence and behavior of groundwater), pedology (soil science), and especially soil-clay mineralogy, geophysical survey (archaeomagnetism with magnetic- and resistivity-force mapping), tephra (volcanitic ejecta—ash, shards), chronology, radiometric dating (radioactive elements and their isotopes, especially carbon), geochemical analyses (including trace element and isotope, useful for provenance), palynology (pollen and spore identity), and floral or plant inventories (including phytoliths), and coastal paleogeography (ancient shore-zone environments and locations). These and other disciplines form the geological framework through which the depositional record of natural and human agencies are interpreted.

Agents of earth sculpture that are active on/or in terrestrial and submarine surfaces include weathering; erosion; movement and deposition of earth materials by means of atmospheric interaction with mineral, rock, and plant material; gravity-slope kinetics; streams; underground water; waves and currents; wind; ice; and even volcanic activity. Vertical land movement and sea-basin volume changes may need to be considered (site elevation, submergence with associated effects). Physiographic surface environments express socio-economic settlement-opportunity options often governed by the demands that specific cultures require.

The topographic configuration of a specific site locale is a function of rock and sediment body resistance to forces of degradation (removal) and aggradation (surface buildup), whose rates of progress determine how quickly human presence alters a surface. The difference between the highest and lowest elevations is mapped as a contour (the interval is determined by the steepness of slope and the amount of detail required). An overlay of geologic rock outcrops affords a convenient means of illustrating those formations that were affected by a site's occupants in construction phases. Other surface environments to be understood in archaeohistorical research include plains, plateaus, mesas, hills, mountains (volcanic, folded, fault-block), canyons, basins (graben or rift valleys), alluvial fans, valleys (stream and glacial), alluvial terraces, floodplains, deltas, beach/shore zones, glacial moraines, till plains, and outwash terrains. Historically, these natural depositions formed human-site contexts in temperate, tropical, arid, and polar settings in which settlers would need to control terrain, water, food acquisition/production, trade, roads, and defensible shelter.

Geologic activities that affect human-settlement decisions include water supply—springs, streams, water tables, and even artesian flow. Geothermal (heated) waters find expression as hot springs. Other water-bearing geochemical activities in the earth's crust generate mineral (ore) deposits (ion concentrations) yielding metals such as copper, tin, iron, lead, and zinc. Certain igneous reactions yield other ore deposits. The earliest metal acquisition by humans involved mining naturally occurring elements. As smelting/refining extraction techniques from the ores of those metalic compounds were discovered, the metals were incorporated into tool and weapon economies, recording the historical results of metallurgy. [See Metals.] Precious metals and stones also occur because of geologic concentration conditions. Such utilitarian metal ores were mined for noble and royal use: jewelry, tableware, and (by the end of the seventh century BCE) coins. The geochemical generation of petroleum and its by-products—bitumin, naptha, and sulfur and the organogenesis of woods, fibers, amber, spices (fragrances), foods, and shell remains—has heavily influenced human behaviors, as recorded in archaeological remains.

Knowledge of the local and regional bedrock—its stratigraphy and structure—gave ancient masons of both stone and mud brick and quarrymen construction resources with which to erect, pave, wall, arch, and bridge for human requirements in urban, domestic, and defensive structures. Some rock types exhibit cutting properties that would produce poor to excellent architectural and sculptural results—cemented beach sands, rock and shell fragments (imprecisely termed poros), versus the finely crystalline limestones employed for Herodian structures in Roman Palestine and white marble for sculpture.

A brief inventory of common construction, building, and sculptural stone utilized throughout the Mediterranean and the Near East consists mainly of limestones, chalks, caliche (such as nari), coquina, travertines, quartz and bioclastic sandstones/conglomerates, quartz (and other lithic) sandstones, chert, various lithic-type conglomerates (such as kurkar), siltstones, marls, shales, alabasters, halites (from Mt. Sedom, a salt-block feature near the southwestern shore of the Dead Sea), granites, diorites, gabbros, lavas (basalt, from which Levantine and Mediterranean millstones were often crafted), and porphyrys, tuffs, slates, schists, gneiss, marbles, quartzites, and greenstones.

No less vital in human history has been the function of clays, formed mostly in the soil-genesis process, in grazing grasses and food crops. The mineralogical agricultural fertility of floodplain environments (river-valley sediments forming terraces) is well known. These clays and clay minerals have historical significance because they were utilized as tokens, tablets, and letters, mud bricks and mortars, terra cottas (sculptures and ovens), tiles (roofing, flooring, water and drainpipes), and, finally, as pottery.

Commonly, analytic studies and procedures in archaeological geology include, sediment-body (locus) identification, working hypotheses as to origin (source and agent), and sediment surface recognition; architectural building-stone type and source; historical geological/geographic site area-surface morphology; water source and management; pottery composition and provenance; evidence of the presence/absence of bedrock soil-cover depth and extent; stone and ore sources and their proximity to a site; clay/mud sources and their employment throughout the site's cultural history; the encouragement of multidisciplinary site and environmental studies (botany, entomology, zoology, and ecology).

Some of the most important geoscientific methods in archaeological research are petrology (e.g., study of rocks, pottery, bricks, roofing, tiles) in hand specimens and microscopic thin-sections to ascertain mineral makeup, texture, and ultimately, provenance; sedimentology (mineral composition and particle size—from clays to boulders), along with hypotheses concerning agency of deposition; ore and gemstone identification and qualitative analysis from wet chemical to X-ray spectroscopy; laser-probe spectroscopy of metals and gems; and core drilling in sediment strata, in- and off-site, to determine shore/harbor boundaries. [See Spectroscopy.] Typical field-geology research tools and equipment serve most of the study requirements. A binocular stereo-zoom microscope of 10–45 × has been found indispensable in the dig house.


GEOLOGY. Scored basalt slabs paving a Roman/Byzantine decumanus at Abila, northern Jordan. Observe the stratified sediment showing the evidence of disoccupation. Both colluvial slope deposits and architectural fragments have covered the street and its sidewalks. Slumping drain cover blocks reveal the drain path. (Courtesy R. G. Bullard)

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GEOLOGY. Calicheated chalk bedrock used as the foundation level for a wall constructed of the same material. Step-ledge quarrying techniques are evident. This quarrying method was employed by the Romans all over the Mediterranean. (Courtesy R. G. Bullard)

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Reuben G. Bullard