Using BGS and OS Maps for Coastal Landscapes

Two maps answer different questions

A coastal map shows more than the boundary between land and sea. Its coastline shape, relief and labels help you recognise landforms. A geological map adds information about the rocks and structures that help explain those landforms.

An Ordnance Survey (OS) map represents the landscape: where features are, their shapes and the surrounding terrain. A British Geological Survey (BGS) geological map shows the distribution of rock units. Used together, they connect an observation — such as a bay between two headlands — to a possible explanation involving differences in rock resistance.

Reading geology before explaining the coast

Begin with the geological map’s legend. Different colours distinguish geological units, not a simple universal ranking from soft to hard. Identify the named unit and its rock type before deciding how it might affect erosion.

Distinguish bedrock, the underlying solid rock, from superficial deposits, the loose material above it. For an explanation of a rocky headland or cliff, bedrock is especially important. Surface deposits may conceal that bedrock rather than replace it.

The BGS Geology Viewer allows you to tap a location to reveal bedrock and superficial geology. Its legend updates as you move around the map; opening the sidebar and selecting ‘View the layer legend’ shows the visible formations. A legend entry also provides a route to fuller information about the unit.

Next, examine where the rock boundaries meet the coastline. Ask whether different units coincide with projecting headlands, recessed bays or changes in coastal steepness. Where structural information is available, consider the direction of the rock bands and any mapped faults. A fault is a fracture along which rock has moved; geological structure can influence where erosion develops.

A worked comparison: coastline shape and rock resistance

The following illustration is a fictional coast, not an OS map or a survey of a real place. It demonstrates how to connect mapped geology with coastal form.

Start with the coastline alone: the middle section curves inland, forming a bay, while the land on either side projects seawards as headlands. Then compare that shape with the geological information. The bay coincides with clay, whereas the headlands coincide with limestone.

The interpretation is that the less resistant clay has been eroded more readily than the more resistant limestone. This helps explain the contrast in coastal shape. It is stronger than simply saying ‘geology affects the coast’ because it connects a particular rock type, a particular landform and a process.

Transfer this method to real maps by identifying the same stretch of coast on both maps and checking the actual geological legend. The illustration demonstrates a relationship, not a rule that every clay coast must contain a bay or every limestone coast must form a headland. Rock structure and coastal processes also matter.

Applying the method to Dorset

Along Dorset’s Jurassic Coast, the Kimmeridge Clay Formation, Wealden Group and Oxford Clay Formation tend to form lower coastal stretches and bays. More resistant units, including the Corallian, Portland, Purbeck and Chalk groups, tend to form steeper stretches, cliffs and headlands.

Structure also helps explain coastal form. In the central Isle of Purbeck, steeply dipping or almost vertical Portland and Purbeck limestones run parallel to the coast. They form a resistant barrier that has been breached in places, producing features including Lulworth Cove and the arch at Durdle Door.

This shows why identifying a rock name is only the first step. The arrangement of the rocks can be just as important as their resistance. BGS mapping that joins onshore and offshore geology helps reveal structures continuing beyond the visible cliffs.

Using scale to judge coastal dimensions

A map scale compares map distance with ground distance using the same units. At 1:25,000, 1 cm on the map represents 25,000 cm on the ground, or 250 m. At 1:50,000, 1 cm represents 500 m. A 1:25,000 map is therefore larger-scale: it can show more detail over a smaller area.

For example, suppose the straight-line distance between two ends of a beach is 3 cm on a correctly sized 1:25,000 map. The ground distance is $3 \times 250 = 750\text{ m}$. This measures the straight-line span, not necessarily the distance along the curved beach. To measure a curve, follow it in short sections and add their lengths before converting to ground distance.

On a resized map image, use its scale bar rather than assuming that a centimetre on your screen still represents 250 m. Also check the scales when comparing OS and BGS maps: matching places matters more than matching their apparent size on the page.

Recognising landforms on an OS map

Read the coastline’s outline first, then check the map’s labels, legend and relief information. Use several clues together rather than relying on a place name alone.

• Headland: land projecting into the sea. Look for a seaward bulge with recessed coastal sections beside it.
• Bay: an indentation in the coast. Trace the shoreline to distinguish the enclosed curve from the projecting land around it.
• Cliff: a steep coastal face. Look for mapped cliff information and a rapid change in height close to the shore. Closely spaced contours indicate steep terrain, but steepness alone does not prove that a cliff is present.
• Beach: a strip of deposited sediment along the shore. Check the legend for mapped sand, shingle or mixed surface material rather than assuming every coastal strip is sandy.
• Spit: a narrow depositional feature attached to land at one end and extending into the sea or across an inlet. Its free end distinguishes it from a bar that joins land at both ends.
• Bar: a depositional ridge joining land across a bay or inlet; enclosed water behind it can provide another clue.

OS 1:25,000 mapping uses blue for water features. Its contours may have 5 m or 10 m intervals, so check the interval on the particular map before interpreting changes in height. At very steep faces, contours can merge or band together. Read these relief clues alongside the coastline and the legend, rather than treating any one clue as a complete identification.

For example, on a map, a narrow strip marked as sand and shingle extends from the mainland into an inlet but stops before reaching the opposite shore. Its sediment information and attachment at only one end support identification as a spit. If a depositional ridge instead joins the two sides of the inlet, it supports identification as a bar. This reasoning transfers to authentic OS maps, but the actual surface symbols must be read from their own legend.

Caves, arches, stacks and wave-cut platforms may need more detailed mapping or additional evidence. A small isolated offshore rock could be a stack, but its outline alone does not establish how it formed. Likewise, a plan-view coastline cannot reliably reveal a cave entrance or show that a rock opening passes all the way through a headland. Use explicit mapped information where available rather than forcing an identification.

Building an evidence-based explanation

A useful comparison follows three stages: identify the landform, identify the geology, explain the connection. For example, recognising a recessed bay is an observation; identifying clay there is geological evidence; explaining that less resistant rock can be eroded more readily connects the two.

Finally, consider scale and detail. A geological layer may remain visible when you zoom far beyond the scale of its underlying data. That does not make a boundary more precise. Maps are valuable evidence for coastal form and geological relationships, but a single map does not measure a current rate of coastal retreat.

Compare OS and BGS maps

• OS: coastline shape, landforms, relief and surface features.
• BGS: rock units, bedrock/superficial deposits and available structural information.
• Method: identify landform → read geological legend → compare location → explain relationship.

Geological links

• Less resistant rock tends to form recessed bays and lower coastal stretches.
• More resistant rock tends to form headlands and steeper, cliffed stretches.
• Rock arrangement matters as well as rock type.
• Dorset: softer clay units contrast with more resistant Corallian, Portland, Purbeck and Chalk units.
• Central Isle of Purbeck: steep limestone bands parallel to the coast form a barrier breached at features including Lulworth Cove and Durdle Door.

Recognise coastal shapes

• Headland: projects seawards; bay: curves inland.
• Cliff: steep coastal face; combine relief with mapped information.
• Beach: shore sediment; check surface information.
• Spit: attached at one end, with a free end.
• Bar: joins land across a bay or inlet.
• Small erosional features may require explicit labels or more detailed evidence.

Scale and relief

• 1:25,000: 1 cm = 250 m; 1:50,000: 1 cm = 500 m.
• Closely spaced contours indicate steep slopes; check the contour interval.
• Use a scale bar on resized images.
• Greater zoom does not create more accurate geology, and coastline shape alone does not establish an erosion rate.