The birth of Chesil Beach

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I was interested in researching coastal erosion along the west coast of the UK, particularly along the Jurassic coast and especially in the West Dorset areas covering both Chesil beach and the coast between Bridport harbour, (West Bay), and Seaton in adjoining Devon.

Primarily, in keeping with my comprehensive studies and interest in Anglo-Romano Dorset, I wanted to know how much coastal erosion might have occured along this stretch of coastline over a 2000 year period. (From Roman times till today), although as I research the topic further, I might want to investigate greater coastline erosion over longer periods of time, say, from modern times back through 11,700 years to the end of the last ice age, a time when Mesolithic Hunter – Gatherers would have ventured across a landbridge from what we know as Europe approxiamately 11,000 years ago.

The rich and varied local geology and topography makes West Dorset a fascination subject of exploration and examination with regards to coastal erosion, particularly along the Jurassic Coast and West Dorse shorelines that represent home to me so this was always going to become a focus of intgerest and would, obviously, expand my local knowledge further.

And sure. The data surrounding the erosion of a 2,000-year period of Dorset coastal history would depend on several variables including rock type, wave energy, storm frequency, sea-level rise, human influence and more but I’d already learned several intriguing facts about both coastal erosion patterns and archaological factors right here on my doorstep so felt this was a subject well worth exploring further.

Jurassic Coast and West Dorset Specifics

  1. Geology:
    • The Jurassic Coast features sedimentary rocks, including limestone, sandstone, shale, and clay, which erode at varying rates. Softer rocks (e.g., clays) erode much faster than harder rocks like limestone.
    • Chesil Beach, a unique shingle ridge, is dynamic and prone to reworking by storms and longshore drift, but not strictly erosion in the traditional sense.
  2. Current Erosion Rates:
    • The cliffs along this area typically erode at rates of 1-3 cm per year for harder rocks like limestone but can exceed 30 cm per year for softer materials like clay during wetter conditions or following storms.
    • Chesil Beach experiences sediment movement rather than direct erosion. However, rising sea levels may impact its configuration significantly.
  3. Projections Over 2,000 Years:
    • Hard Cliffs (e.g., limestone cliffs): Approximately 20–60 meters of retreat, assuming current erosion rates remain constant.
    • Soft Cliffs (e.g., clay and shale): Potentially 200–600 meters of retreat, or more in areas prone to landslides.
    • Chesil Beach: Its position and form could shift inland or fragment, depending on sea-level rise and storm frequency.

Key Influencing Factors

  1. Sea-Level Rise:
    • Predicted global average rise is about 1 meter by 2100 under worst-case scenarios, accelerating erosion.
    • Over 2,000 years, a rise of several meters could be plausible, significantly affecting low-lying areas and increasing erosion.
  2. Storm Frequency and Intensity:
    • If storm activity increases, erosion rates could rise, especially for softer cliffs and vulnerable formations.
  3. Human Activities:
    • Coastal defenses (e.g., groynes, sea walls) could slow erosion locally but exacerbate it elsewhere due to sediment starvation.
  4. Long-Term Perspective (11,700 Years):
    • Since the end of the last Ice Age, the UK coastline has changed drastically. Sea levels rose approximately 120 meters since the Last Glacial Maximum (~20,000 years ago), submerging large coastal areas.
    • For West Dorset, long-term processes have resulted in the creation of features like the Lulworth Cove and Chesil Beach, both highly dynamic and shaped by wave action and sediment supply.

Research Opportunities

  • Modeling and Projections: Use historical erosion rates combined with predictive models accounting for climate change.
  • Local Historical Records: Old maps and accounts of coastline positions can offer insights into erosion trends.
  • Field Studies: Assess erosion scars, sediment deposits, and landslide activity.

Would you like help finding current studies, mapping tools, or predictive erosion models for this region?

4o

You said:

Yes but before we consider your suggestions, please investigate how, prior to the last ice age dissapating, the shoreline was further out to sea than where the newer Chesil beach manifest and please confirm whether the original shoreline actually ran level with where the tip of Portland Island currently sticks out into the English channel at present.

ChatGPT said:

ChatGPT

The shoreline in the region of West Dorset and Chesil Beach has indeed undergone significant changes due to fluctuating sea levels before, during, and after the last Ice Age. During the Last Glacial Maximum (LGM) around 20,000 years ago, global sea levels were approximately 120 meters lower than today. This meant the shoreline was significantly further out to sea, including in the area of the present-day Jurassic Coast.

Key Points about the Ancient Shoreline:

  1. Shoreline Extension During the LGM:
    • At the LGM, the English Channel was largely dry land, and the shoreline was much farther out to sea.
    • The Portland Stone that forms the island was part of a continuous landmass rather than an isolated feature.
    • Based on bathymetric data and studies, the shoreline likely extended several kilometers south of where Chesil Beach currently exists, beyond the Isle of Portland.
  2. Alignment with Portland’s Tip:
    • Geological evidence suggests that the ancient shoreline ran roughly level with or even beyond the current tip of Portland Island. The exact positioning would have depended on the topography of the sea floor and sediment deposition patterns at the time.
    • The presence of submerged river valleys and ancient sediment deposits indicates that rivers would have flowed across this exposed plain, discharging further out into the English Channel.
  3. Formation of Chesil Beach:
    • As glaciers melted and sea levels rose post-Ice Age (starting about 18,000 years ago), sediment from rivers and coastal erosion was mobilized by waves and longshore drift, forming features like Chesil Beach.
    • Chesil Beach is primarily composed of pebbles derived from cliffs to the west. Rising seas pushed these sediments inland, eventually creating the shingle ridge we see today.
  4. Portland as a Promontory:
    • Portland Island was once part of the mainland before being isolated by rising sea levels and the formation of the Fleet lagoon and Chesil Beach. During earlier periods of lower sea levels, it would have been an inland feature or a coastal promontory.

Confirming the Original Shoreline:

Studies of submerged landscapes in the English Channel, including sonar mapping and sediment analysis, corroborate that the shoreline extended beyond Portland’s tip. Archaeological finds and submerged river systems also provide clues about the ancient geography:

  • The English Channel acted as a land bridge connecting Britain to mainland Europe until about 8,000 years ago.
  • Submerged sediment beds near Portland and Chesil Beach have been studied to understand sediment transport and ancient coastal configurations.

If you’d like, I can help locate specific studies, bathymetric maps, or geological surveys that detail this prehistoric landscape further.