Coastlines of emergence

Overview

Coastline of emergence, raised beach and relict cliff.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What is a coastline of emergence?

   Model answer

   A coastline where relative sea level has fallen, so that features formed at or below the former sea level are now exposed above it. It is produced either by isostatic uplift of the land, most commonly the slow recovery of crust that was depressed by an ice sheet, or by a eustatic fall in the volume of ocean water, or by tectonic uplift. Its characteristic signature is a coast with old marine landforms stranded above the present shore and, on lowland coasts, a broad flat plain of former sea bed.

2. 2

   What is a raised beach?

   Model answer

   A former beach, complete with its rounded shingle, sand and shell deposits, now lying above the reach of the present tides, usually as a flat or gently sloping terrace running along the coast at a consistent height. It was formed at the shoreline when relative sea level was higher, and it has been left stranded by the subsequent uplift of the land or fall of the sea. Because it is no longer reached by waves, it is dry, often grassed over or cultivated, and it frequently carries a road, a railway or a line of settlement, since it is the only flat land along an otherwise steep coast.

3. 3

   What is a relict cliff, and what features are found on and around it?

   Model answer

   A relict cliff, sometimes called a fossil or degraded cliff, is a former sea cliff standing at the back of a raised beach, now well inland of the modern shoreline and beyond the reach of the waves that cut it. Because marine erosion no longer attacks its base, it is no longer maintained as a steep face: subaerial weathering and mass movement gradually reduce its angle, scree accumulates at its foot, and vegetation and even woodland colonise it, so it becomes a rounded, grassed-over slope rather than a bare rock face. Along it can be found relict marine features that are unmistakably the work of the sea: wave-cut notches, caves now dry and above the beach, and occasionally arches and stacks standing inland on the raised platform.

4. 4

   How would you recognise from the field evidence that a cliff was relict rather than active?

   Model answer

   Its base is not being reached by the sea, and there is a raised beach or flat terrace between it and the present shoreline. Its face is degraded rather than steep and fresh, with a lower angle, an accumulation of scree or slumped material at the foot, and a cover of grass, scrub or trees. Any caves in it are dry, floored with soil or scree rather than with wet shingle, and free of marine organisms. Rounded, well-sorted beach shingle, shells and sometimes marine fossils are found on the terrace in front of it, plainly of marine origin but well above the tide. The combination of unambiguously marine forms and deposits with an absence of any current marine activity is the diagnostic evidence.

5. 5

   What other features occur on coastlines of emergence?

   Model answer

   Raised wave-cut platforms, the exposed rock benches on which the raised beach deposits usually rest, sometimes forming a flight of several at different heights that record successive stages of uplift. Raised mudflats and marine terraces. On gently sloping lowland coasts, a coastal plain of emergence, a wide flat area of former sea bed now above water, often poorly drained and with old shoreline ridges running across it. Raised deltas, and rivers that have cut down through their own former floodplains because base level has fallen. And stacks and arches stranded above and behind the modern shoreline.

6. 6

   Why can a coast show a flight of several raised beaches at different heights?

   Model answer

   Because uplift has not been continuous and even. Isostatic recovery proceeds in stages, and periods of relatively rapid uplift alternate with periods of stability. During each still-stand, the sea has time to cut a platform and build a beach at that level. When uplift resumes, that shoreline is lifted clear and a new one begins to form at the new lower level. Each terrace therefore records a period when relative sea level stood at that height, and the highest is the oldest. The heights are not identical from place to place along a coast, because the amount of isostatic recovery varies with how much ice the crust once carried, which is itself good evidence that the cause was isostatic rather than eustatic.

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