Using BGS and OS Maps for Coastal Landscapes

Geological Structure: Concordant vs Discordant

• The way rock layers are arranged relative to the coastline is called geological structure.
• A discordant coastline occurs when rock strata (bands) run perpendicular (at right angles) to the coast. This exposes alternating bands of hard and soft rock to the sea simultaneously.
• Differential erosion along a discordant coast creates a series of headlands and bays. A classic example is the East Coast of the Isle of Purbeck, where soft Wealden Clay forms Swanage Bay, while hard Chalk forms a headland called The Foreland.
• As waves approach a discordant coast, wave refraction occurs. The waves bend toward the shallow water of the headlands, concentrating their erosive energy there, while dissipating energy in the bays to encourage beach deposition.
• A concordant coastline occurs when rock strata run parallel to the coast. A single resistant rock layer faces the sea, creating a straight coastline that protects the softer layers behind it, such as on the South Coast of the Isle of Purbeck.

Worked Example: Formation of a Cove

Explain the causal mechanism that leads to the formation of a cove on a concordant coastline.

Step 1: Identify the initial structure.

• A concordant coastline features a highly resistant outer layer of rock (e.g., Portland Limestone) running parallel to the sea, protecting less resistant rock behind it.

Step 2: Explain the initial breach.

• Marine processes exploit a structural weakness, such as a fault or joint, in the hard outer rock until the sea breaches it.

Step 3: Explain the role of differential erosion.

• Once the sea reaches the softer rock behind (e.g., Wealden Clay), differential erosion occurs. The soft rock erodes significantly faster than the hard rock.

Step 4: Describe the final shape.

• Wave refraction within the newly formed breach causes erosion to spread laterally (sideways), creating a circular or horseshoe-shaped cove, such as Lulworth Cove.

The Role of Rock Dip

• The angle at which rock layers are tilted from the horizontal plane is called the dip. This determines the cliff profile (cross-section shape) and its stability.
• Seaward dip: Rock strata tilt toward the sea. If the angle is high ($>45^\circ$), rocks easily slide down the bedding planes into the sea, making this the most unstable profile with a high risk of mass movement.
• Landward dip: Rock strata tilt away from the sea. Gravity pulls loosened rocks back into the cliff face, making this steep profile (approx. $70^\circ$ to $80^\circ$) highly stable.
• Horizontal dip: Rock strata are laid flat. This forms vertical cliffs that are prone to undercutting at the base, leading to eventual blockfall.

Identifying Coastal Landforms on OS Maps

• To locate specific features, geographers use grid references. A 4-figure grid reference identifies a $1$ km$^2$ square, while a 6-figure grid reference provides a precise location to within $100$ m.
• Cliffs: Identified by very closely spaced contour lines and black hachures (short perpendicular tick-marks) indicating a steep face.
• Stacks and Stumps: Stacks appear as isolated black shapes detached from the mainland; stumps are smaller and often only visible at low tide.
• Wave-cut platforms: These appear as jagged, "bumpy" black lines or "flat rock" symbols seaward of the cliff base, typically located between the High and Low Water marks.
• Beaches: Found in the intertidal zone, between the Mean High Water (MHW) and Mean Low Water (MLW) blue lines. OS maps use specific shading to identify beach material: yellow shading indicates sand, while yellow with black speckles/dots indicates shingle or pebbles.

Comparison of OS Map Scales

Feature	1:25,000 (Explorer)	1:50,000 (Landranger)
Scale	$4$ cm on map $= 1$ km	$2$ cm on map $= 1$ km
Relief	$5$ m contour intervals (precise)

Linking Coastal Form to Geology (BGS Maps)

To explain why a coast looks the way it does, geographers use BGS Geological Maps to identify the underlying rock types and structures.

BGS Map Features:

• Color-Coding: Bold/dark colors (pinks/reds) represent resistant igneous rocks like Granite. Lighter shades represent younger, less resistant sedimentary rocks like Clay.
• Letter Codes: Rocks are labeled with shorthand, e.g., C (Chalk), g (Granite), WC (Wealden Clay), POST (Portland Limestone).
• The Key: Presented in stratigraphic order, with the oldest rocks at the bottom and youngest at the top.
• Structural Weaknesses: Thick black lines indicate faults. Arrows pointing away from an axis show an anticline (upward fold), while arrows pointing inwards show a syncline (downward fold).

BGS Map Analysis Step-by-Step

1. Locate the Color/Code: Find the rock type on the BGS key (e.g., Code 'WC' = Wealden Clay).
2. Determine Resistance: Identify if the rock is resistant (Hard) or non-resistant (Soft). (e.g., Clay is less resistant).
3. Link to OS Form (Dual Evidence): Explain how this geology creates the landform seen on the OS map.
   • Example: "The BGS map shows less resistant Wealden Clay (WC) in this area. This matches the OS map evidence of an indented bay and widely spaced contours, as the soft rock has eroded rapidly via differential erosion."