Investigative Geology: Maps, Fieldwork and Practical Skills
This lesson covers general fieldwork skills required by the spec. Your school will have done specific fieldwork at a specific location — refer to your teacher's notes for your own examples and case-study data.
The Simplified Geological Map
Your written examination (the practical-skills component) is entirely based on a simplified geological map. Unlike the full 1:50,000 BGS maps used in professional work, the exam version uses a small number of named rock units, shows faults and fold axes clearly, and is accompanied by a cross-section framework and a stratigraphic column. The skills you need are: reading the map to identify rock types at any point, inferring structure from the map pattern, drawing or completing a cross-section, and constructing a sequence of geological events from the evidence shown.
Map reading is not passive. When you look at a simplified geological map, you should immediately ask: are the rock unit boundaries roughly parallel to the contours (horizontal strata), or do they cross contours (indicating dipping strata)? Are there any V-shaped bends in boundaries where a stream crosses them — and does the V point upstream (strata dipping upstream, or horizontal strata following the contours) or downstream (strata dipping downstream)? These V-patterns, combined with knowing the rule of Vs, allow you to infer the dip direction of strata before any other information is given.
The rule of Vs: when a geological boundary crosses a valley, the V points in the direction of dip. Horizontal strata follow the contours, so their V points upstream. Strata dipping upstream produce a V pointing upstream — sharper than the contour V. Strata dipping downstream produce a V pointing downstream. Vertical strata show no V at all — the boundary crosses the valley in a straight line.
Constructing a Cross-Section
A geological cross-section shows what the geology looks like on a vertical slice through the Earth along a given line on the map. To construct one:
First, draw the topographic profile — plot the elevation at each point where a contour crosses your section line, then join the dots with a smooth curve. Next, mark where each geological boundary crosses the section line at the surface. Use the dip angles and directions given on the map (shown as dip arrows with the angle written beside them) to project boundaries downward into the subsurface at the correct angle. In exam answers, show the beds as parallel lines within each rock unit, continuing the boundary angles consistently. Faults are drawn as straight lines cutting across bedding at the correct dip, with the displacement clearly shown.
Constructing a geological history from a map is one of the highest-tariff skills in the practical-skills component. The method uses three principles in sequence: superposition (older rocks are below younger ones in an undisturbed sequence), cross-cutting relationships (any feature that cuts across another is younger: a fault cutting through bedding formed after the bedding; a dyke intruding across a fault formed after the fault), and unconformities (a surface of erosion representing a gap in the geological record, with younger horizontal beds resting on older tilted beds). Exam technique: state the events in order from oldest to youngest, use the names of the specific rock units shown on the map, and state the principle of evidence for each step. Do not just describe what you see — you must explain how the evidence leads to the conclusion.
Graphic Logs
A graphic log (or sedimentary log) is a vertical column drawn to scale showing the rock types and thicknesses encountered in a borehole or at an outcrop. Each rock type uses a standard symbol: dots for sandstone, dashes for mudstone, brick-pattern for limestone, and so on. Additional columns to the right record grain size (coarsening-up or fining-up sequences), fossils found at each level, and sedimentary structures such as cross-bedding or ripple marks. Reading a graphic log allows you to reconstruct the environment of deposition — alternating sandstones and mudstones suggest a fluctuating energy environment such as a tidal flat or delta margin; a thick uninterrupted limestone suggests a stable shallow marine environment.
In the field, a compass clinometer measures the dip angle and strike direction of rock surfaces such as bedding planes or fault planes. Strike is the compass direction of a horizontal line on the surface; dip is the maximum angle of inclination, measured perpendicular to strike. These values are plotted as dip arrows on a geological map. GPS is used to record the precise location of observations, ensuring measurements can be placed accurately on the map.
A rose diagram is a circular graph used to display directional data — for example, the strike directions of joints measured across an outcrop, or the orientation of pebble long-axes in a conglomerate. Petals on the rose represent the frequency of measurements in each compass direction. A rose diagram with two opposite petals suggests a single dominant joint set; one with four petals suggests two conjugate joint sets related to a tectonic stress field. In your exam, you may be asked to read a rose diagram and describe the dominant trend, or to explain what the pattern reveals about past stress or current flow directions.
Rock description in the field uses standardised language: for igneous rocks — colour, grain size (fine/medium/coarse), texture (porphyritic, vesicular, glassy), mineralogy (quartz, feldspar, mica as visible crystals). For sedimentary rocks — grain size, sorting, rounding, colour, sedimentary structures, fossil content. For metamorphic rocks — foliation (slate, schist) or banding (gneiss, but note gneiss is not on the spec; if in doubt, describe the texture without naming). Using only rock names that appear on the specification is essential for exam marks.
The Fieldwork Statement and Directed Investigation
The specification requires two field occasions before the exam, at least one of which must be a directed investigation — where you formulate a question, collect data systematically, and interpret your findings. There is no submitted portfolio; the fieldwork prepares you for examination questions that ask about field methods, data collection and interpretation. You may be asked to describe how you would carry out a field survey, what equipment you would use, how you would record data (field sketches, annotated drawings, written descriptions, graphic logs), and how you would process and present it (rose diagrams, cross-sections, geological histories).
Key fieldwork recording methods: field sketch (hand-drawn panoramic view of an outcrop with annotations for rock types, structures and features); annotated photograph (scaled photograph with labels identifying geological features); graphic log (scaled vertical column showing rock sequence and thicknesses); rose diagram (circular frequency plot of directional data). Each serves a different purpose and you may be asked to select and justify the most appropriate method for a given task.
The Geological Code of Conduct
All geological fieldwork in the UK is guided by The Geological Code of Conduct. Key principles include: always seeking permission from landowners before entering private land; working safely — never hammering at the base of an unstable face; collecting specimens minimally and only where legal; respecting other users of the countryside; and reporting significant new finds to geological bodies or local record centres. In your exam, if asked about responsible fieldwork, cover permission, safety and minimal impact.