Map Skills and Geological Interpretation

Different maps answer different questions

A map represents a place as seen from above. However, maps select different information. An Ordnance Survey (OS) map shows landscape features such as rivers, roads and buildings. A geological map shows the distribution of rocks. A geological cross-section adds a side-on view of the rocks beneath the land.

Reading these representations together helps answer two connected questions: what is the landscape like, and how might its geology help explain it? Relief means the height and shape of the land; geology means its rocks and their arrangement.

Reading a geological map

Begin with the key. Colours or patterns identify different rocks, so a change in colour represents a change in the mapped geology, not necessarily a change in height. Follow the boundaries to see where each rock occurs and whether it forms a broad area or a narrow band.

Next, compare the rock distribution with information about relief. More resistant rocks tend to withstand erosion better and remain as higher ground. Less resistant rocks tend to be worn down more readily, helping to produce lower ground. This is a tendency, not a rule that every hard rock must form a mountain: the arrangement of the rocks and the landscape’s history also matter.

For example, resistant slate occurs in the mountains of North Wales. Limestone can also form upland landscapes, so sedimentary rock should not automatically be treated as lowland rock. Clay, by contrast, commonly forms broad, relatively flat lowland landscapes.

Connecting a map with a geological cross-section

A geological map gives a plan view, while a cross-section shows what lies beneath a chosen line, often labelled A–B. Find the two ends of that line first, then read the section in the same direction. Its upper edge represents the land surface; the areas underneath represent the rocks. Match their colours or patterns to the key and to the rocks crossed by the section line on the map.

In this constructed illustration, resistant rock occupies the higher ground and less resistant rock occupies the lower ground. On the plan view, the rock boundary separates the two surface areas. On the section, you can compare that boundary with the change in land height. A supported interpretation would be: the resistant rock has remained as higher ground because it has been worn down less readily than the neighbouring rock.

The illustration demonstrates a method, not a real UK location or survey. On an actual geological map and section, use the supplied rock names, boundaries and height information rather than assuming the same simple arrangement. Read the shape of the rock layers as well as the surface: a geological basin describes the arrangement of rocks, not simply a hollow in the land surface.

Geology can also help explain drainage. Permeable limestone allows water to pass into the rock, so limestone uplands may have few surface streams. Impermeable clay prevents water passing through easily, helping to produce wetter lowland surfaces. Resistance and permeability describe different properties: one concerns wearing away, the other water movement.

Locating uplands, a lowland basin and rivers

An outline map removes local detail so that the broad pattern is easier to see. First identify the countries and the direction of north. Then distinguish upland regions, lowland basins and rivers rather than treating every physical feature as a single point.

The Scottish Highlands, Lake District and mountains of North Wales are examples of UK uplands. Locate the Highlands in northern Scotland, the Lake District in north-west England and the mountains of North Wales in the northern part of Wales. Broadly, upland landscapes are associated with the north and west, while many lowland landscapes occur in southern, south-eastern and central England.

The London Basin provides a concrete lowland example. It underlies London and a broad area of south-east England, extending eastwards towards the North Sea. Its western end lies around Marlborough in Wiltshire. The Chilterns form part of its northern margin and the North Downs part of its southern margin. On an outline map, label a broad region around London and extending eastwards, rather than marking only the city as a dot.

Its geology also illustrates how to connect map and section evidence. Chalk is exposed around the margins, while younger sedimentary rocks cover it towards the centre; London Clay forms much of the surface geology. A section through the basin can therefore reveal underlying chalk that is not exposed everywhere on the geological map.

The River Thames mainly drains the London Basin. To locate its course, trace it through Reading and London towards its estuary on the south-east coast. The River Kennet runs from the Marlborough area and joins the Thames at Reading. These named places provide anchors for positioning the rivers: represent their courses as lines, not isolated dots.

A geological basin and a river drainage basin are different. The Thames also drains areas outside the London Basin, so the geological boundary is not the boundary of all land drained by the river. When transferring these features to an outline map, check positions against the coastline and neighbouring regions. A small outline map shows their broad location, not every bend or precise geological boundary.

Recognising physical and human features on OS maps

OS maps use shapes, colours and symbols to convey information without covering the map in descriptions. The key gives each symbol’s meaning. Identify a feature first, classify it, then describe its relationship to nearby features.

Physical features include rivers, lakes, cliffs, scree and the shape of the land. Rivers appear as blue lines and larger areas of water as pale blue areas. Cliff markings identify steep rock faces; scree markings indicate loose rock fragments. Contours show relief rather than a separate object on the ground.

Human features include roads, buildings, settlements, railways and tourist facilities. On both scales, a blue tent symbol identifies a campsite, a caravan symbol identifies a caravan site, and a white P on a blue background identifies parking. Buildings are shown by small building shapes. Place-of-worship symbols distinguish features such as a tower or a spire, minaret or dome, so their precise form matters.

At 1:50,000, the road key distinguishes blue motorways, green primary routes, magenta main roads and orange secondary roads. Railway lines are black, with separate station symbols. Magenta dotted lines identify public footpaths, whereas a black dashed path marking does not itself establish a public right of way. Public-right-of-way markings are not shown on OS maps of Scotland, where responsible access rights apply.

Woodland needs careful interpretation. Coniferous tree symbols have a pointed form; non-coniferous tree symbols have a rounded crown. The 1:50,000 key also identifies mixed woodland using both types. Trees are a landscape feature, but their presence alone does not establish whether the woodland is natural or planted. Similarly, a reservoir contains water but is a human-made feature; Resr is an abbreviation used on 1:25,000 maps.

At Hay-on-Wye, OS mapping shows the river beside the settlement, with roads and buildings concentrated around the town. Reading the river and settlement together gives a spatial relationship, not just two separate feature names. It does not, by itself, prove why the settlement developed there.

Reading height and slope from contours

A contour joins points at the same height above mean sea level. Its number gives that height in metres. A spot height gives the height of a particular point, rather than a whole line.

Contour spacing reveals slope. Closely spaced contours mean a large height change over a short horizontal distance: a steep slope. Widely spaced contours mean the same height change occurs over a longer distance: a gentler slope. Closed contours with heights increasing towards the centre indicate a hill.

Check the contour interval, the vertical difference between adjacent lines. On 1:25,000 OS maps it is 5 m and/or 10 m; on 1:50,000 maps it is 10 m. For example, with a 10 m interval, a labelled 100 m contour followed uphill by two successive contours gives heights of 110 m and 120 m. This illustrates how to read unlabelled lines; the actual interval must come from the map you are using.

Combine this with geological evidence. If resistant rock coincides with high contour values, while neighbouring clay coincides with lower values, you have evidence for a geology–relief relationship. Height values establish which area is higher; spacing establishes which slopes are steeper.

Working at 1:25,000 and 1:50,000

A ratio scale uses the same units on both sides. At 1:25,000, 1 cm on the map represents 25,000 cm, or 250 m, on the ground. At 1:50,000, 1 cm represents 500 m.

For a constructed comparison, a straight ground distance of 1 km would occupy 4 cm at 1:25,000 but only 2 cm at 1:50,000. The place has not changed; its representation has become smaller. The 1:25,000 scale allows more local detail to be shown, while 1:50,000 covers more ground within the same map dimensions.

Use the key belonging to the particular map and scale. Recognising a feature depends on its symbol and context, not merely on how large it appears.

Geology and relief

• Read the geological key and boundaries; match a cross-section’s endpoints to its map line.
• More resistant rock tends to form higher ground; less resistant rock tends to form lower ground.
• Resistance concerns erosion; permeability concerns water movement. Limestone can form uplands.
• Combine named rocks with height or slope evidence.

UK locations

• Scottish Highlands: northern Scotland.
• Lake District: north-west England.
• North Wales mountains: northern Wales.
• London Basin: London and south-east England, extending towards the North Sea; chalk margins include the Chilterns and North Downs.
• Thames: Reading → London → south-east coast estuary.
• Kennet: Marlborough area → joins Thames at Reading.
• Mark regions as areas and rivers as courses. Geological and drainage basins need not coincide.

OS recognition and relief

• Physical: rivers, lakes, cliffs, scree and relief.
• Human: buildings, roads, railways, campsites and parking.
• Use the scale-specific key; identify features before describing spatial relationships.
• Close contours = steep; wide spacing = gentle. Spot heights refer to individual points.
• Contour intervals: 1:25,000: 5 m and/or 10 m; 1:50,000: 10 m.

Scale conversions

• 1:25,000: 1 cm = 250 m; 4 cm = 1 km.
• 1:50,000: 1 cm = 500 m; 2 cm = 1 km.