Identifying Glaciated Landforms on OS Maps

Turning a map into a landscape

An Ordnance Survey (OS) map shows a landscape from above. Recognising glaciated upland landforms means using its information to imagine the shape of the ground: a hollow beneath a summit, a narrow ridge or a broad valley with steep sides. The strongest identification combines several clues rather than relying on a place name or a lake alone.

Contours join points of equal height above sea level. Follow their height values to establish which way the ground rises. Closely spaced contours indicate a steep slope because height changes over a short horizontal distance; widely spaced contours indicate a gentler slope. Closed contours with heights increasing towards the centre indicate a hill or summit.

Check the contour interval, the height difference between successive contours, in the map key. On OS maps it is commonly 10 m, but some maps use 5 m in flatter areas. Not every contour is numbered, so use the interval to work out intermediate heights. For example, if the interval is 10 m, successive contours uphill from a labelled 250 m line represent 260 m, 270 m, 280 m and so on. The tops of OS contour numbers point uphill, providing another clue to slope direction.

Working at the two OS scales

At 1:25,000, 1 cm on the map represents 250 m on the ground. At 1:50,000, 1 cm represents 500 m. A feature therefore appears twice as large on a 1:25,000 map as on a 1:50,000 map printed at its intended size.

For example, a valley section measuring 4 cm on a 1:25,000 map represents 1 km. The same ground distance measures 2 cm at 1:50,000. The larger-scale map helps you examine smaller features, while the smaller-scale map helps you see how valleys, ridges and summits fit together. Use the scale bar if an extract has been enlarged or reduced.

Distinguishing valleys from ridges

Contour lines bend as they cross valleys and projecting ridges called spurs. Where they form a V, a V pointing uphill indicates a valley; a V pointing downhill indicates a spur. Check the heights to establish which direction is uphill. A stream following the low ground can help confirm that you are looking at a valley rather than a ridge.

This distinction matters because the same curved lines can look like a hollow or a hill if you read the heights in reverse. First establish where the high and low ground lies, then interpret its shape.

Finding corries, tarns and sharp ridges

A corrie is an armchair-shaped hollow with a steep back wall, a basin and a lip at its outlet. On a map, look for contours curving around a hollow, closely packed along its back and sides. Establish from the heights that the ground falls into the hollow rather than rising towards a hilltop. A small lake within it supports the interpretation: this is a tarn, such as Red Tarn beneath Helvellyn in the Lake District.

An arête is a narrow ridge with steep slopes on both sides. Trace the high ground separating two hollows or valleys, then look for close contours descending away from either side. Striding Edge in the Lake District is an example. The important clue is the ridge's relationship to the neighbouring hollows, not simply its height.

A pyramidal peak is a pointed summit formed where three or more corries erode back towards one another. Look for contours closing around a high summit and ridges extending between the surrounding corries. Closed summit contours alone show a hill; the surrounding pattern provides evidence for a glacial peak.

Recognising glacial valleys

A glacial trough has a broad, relatively flat floor and steep sides. On a map, look for closely packed contours along both valley sides, with a broad low area between them. There may be few contours across the nearly level floor. This pattern suggests a U-shaped cross-section, although the map itself is a view from above.

A long, narrow lake lying along the valley floor may be a ribbon lake. Windermere is an example. Its elongated shape and position within a glacial valley are more useful evidence than the presence of water by itself.

A hanging valley joins the main trough from above. Trace a smaller tributary valley towards the main valley and compare their floor heights near the junction. A marked drop between them supports the identification; a stream may descend over a waterfall. The smaller tributary glacier eroded less deeply than the larger glacier in the main valley.

Truncated spurs are ridges cut short along the sides of a glacial trough. Look for projecting high ground ending in a steep face beside the main valley, rather than continuing across its floor. Several such steep-ended ridges can help explain why the trough follows a relatively straight course.

Putting the clues together

Consider a fictional map-reading example, not a real OS location. A small lake lies in a hollow surrounded on three sides by closely packed height contours. The contours rise from 600 m near the lake to 800 m behind it, and the hollow opens towards lower ground on its fourth side. Together, these clues suggest a corrie containing a tarn: a basin beneath a steep back wall, with an outlet towards lower land.

Now suppose a narrow strip of high ground separates this hollow from another hollow, with steep slopes descending on both sides. That additional relationship suggests an arête. It is the combination of height, shape and position that makes the interpretation convincing.

The same reasoning transfers to real OS maps at both scales: establish heights, trace the shape, then check neighbouring features. Real contours are less tidy, and features smaller than the contour interval may not be represented by contour lines. Use the map key to interpret any additional symbols.

A map cannot by itself reveal every detail of the rock or sediment. For example, elongated closed contours may suggest a drumlin, especially where several similar hills occur together, but contours alone cannot prove that a hill consists of glacial till. Map evidence supports an interpretation rather than automatically proving how every feature formed.