2.1 Field methods, sampling & safety

The field geologist's toolkit

Field geology is the disciplined practice of observing, measuring, and recording rock and sediment relationships in their natural setting, then translating those observations into defensible data. The ASBOG Fundamentals of Geology (FG) exam expects fluency with standard field instruments and the reproducible procedures that make observations trustworthy. The workhorse tool is the Brunton compass, or pocket transit, which integrates a magnetic compass, a bubble level, and a clinometer in a single instrument. With it a geologist measures directions (bearings or azimuths), horizontal angles, and the inclination of planes or lines. Before any bearing is trusted, the compass must be set for magnetic declination — the local angle between true (geographic) north and magnetic north — or every azimuth will be systematically offset by several degrees. Declination varies with location and drifts slowly over time, so it is checked against a current map or model.

Measuring strike and dip

The single most important field measurement is the orientation, or attitude, of a planar feature such as bedding, a joint, a fault, or metamorphic foliation. Attitude is recorded as strike and dip:

• Strike is the compass bearing of the horizontal line formed where the inclined plane intersects a horizontal surface. You level the compass edge against the plane and read the azimuth — for example N40°E, written 040°.
• Dip is the angle of maximum inclination of the plane below horizontal, measured perpendicular to strike, and it is paired with the dip direction (the quadrant toward which the plane descends). A bed might be logged "N40°E, 30°SE."

Because dip is measured perpendicular to strike, the two directions are always at right angles. The steepest possible inclination is the true dip; any oblique measurement yields a shallower apparent dip, a distinction that matters when constructing cross-sections. For linear features — fold hinges, slickenlines, mineral lineations — geologists instead record trend and plunge, the compass bearing of the line and its downward angle from horizontal. A consistent quadrant or right-hand-rule convention keeps records unambiguous.

Describing an outcrop

A useful outcrop description is systematic, quantitative, and repeatable. Geologists record rock type; color when fresh and when weathered; grain size, sorting, and rounding; mineralogy; sedimentary, igneous, or metamorphic textures; bedding or layer thickness; and the nature of the contacts between units (sharp, gradational, or erosional). Structural measurements, fossils, weathering features, and evidence of deformation are logged with their station locations. Field sketches and photographs should include a scale object — rock hammer, lens cap, coin, or graduated scale bar — and a north arrow. Consistent, quantitative notes let a second geologist relocate the station and reproduce the interpretation, which is the essence of professional practice.

Sampling protocols

Samples must be representative of the unit and thoroughly documented. Best practice is to collect fresh, unweathered material, which usually means breaking through the weathering rind with a hammer, unless the weathering itself is the subject of study. Each sample receives a unique station number, and its precise location, orientation (for oriented samples used in paleomagnetism or fabric analysis), lithology, and collection date are entered in the field notebook. For geochemical work, cross-contamination is avoided by cleaning tools between sites, using clean containers, and recording chain-of-custody for environmental or forensic samples. Water and soil samples in environmental investigations follow prescribed sample volumes, preservation (chilling, acidification), and holding times defined by regulatory methods. Systematic grid or traverse sampling reduces the bias inherent in grabbing whatever material is most convenient.

Locating stations: GPS and maps

Modern fieldwork fixes locations with a handheld GPS receiver, recording latitude and longitude or UTM coordinates, typically within a few meters. GPS accuracy degrades under forest canopy or in deep canyons, so geologists still confirm position against a topographic map using landmarks and contour patterns. The coordinate datum (for example NAD83 versus WGS84) and the elevation must be logged, because mismatched datums can shift a plotted position by tens to hundreds of meters and corrupt a map.

Field safety

Safety is a professional and ethical obligation, and ASBOG treats sound field conduct as part of competent practice. Core precautions include:

Additional rules round out a safe program: check the weather and flash-flood risk before entering canyons or dry washes; obtain landowner permission and respect access law; carry adequate water, food, map, compass, and first aid; and never work alone in truly remote terrain without a check-in schedule. Quarries, mines, and active construction sites impose their own regulated personal protective equipment (PPE) requirements. The licensed geologist bears responsibility for the safety of any crew under their supervision. Together, precise instrumentation, systematic description, representative sampling, accurate location control, and disciplined safety define competent field practice — the foundation on which every later interpretation rests.