Geology
The layers of sands, gravels and clays that make up Hengistbury Head were formed around 65 million years ago beneath a warm tropical sea.
This was the beginning of a very unstable period. Movements in the earth’s crust created the Alps and in southern England the land surface was gently folded. Temperatures and sea levels fluctuated widely. Tropical seas gave way to warm lagoons and then extreme cold as a series of ice ages gripped the land.
This dramatic history is visible in the cliffs along Hengistbury Head.
Key
1. Ancient Tropical Sea
Boscombe SandsThe oldest visible part of the headland was laid down between 65 and 46.5 million years ago under a warm tropical sea. The remains of tropical plants, trapped in the sand, give this layer its purplish-black colour.
2. River Lagoons
Lower Hengistbury Beds
These olive green, sandy clays were laid down between 46.5 and 42.5 million years ago. Trapped within them are the remains of plants that grew in sub-tropical river lagoons, similar to the Florida Everglades.
3. Ironstone Armour
Upper Hengistbury Beds
These greenish, sandy clays contain one of Hengistbury Head’s most important features. Between 42.5 and 40.5 million years ago the land was drowned under a large river estuary. A mineral called siderite (iron carbonate) mixed with the estuary sands and clays to form ironstone known as ‘doggers'.
4. On the Sea Shore
Warren Hill Sands
These bands of yellow and white sand are evidence of a beach that was here 40 million years ago; and today - an important site for nesting sand martins.
5. Ice Age Floods
Between 1.8 million and 12,000 years ago the earth experienced a series of ice ages. During warmer interglacial periods a melt-water river gushed from the edge of the ice sheet, leaving these sands and gravels in its wake.
6. Rocks of the Future
Wind-blown sand and soil has been building up on the top of the headland for the last 12,000 years. If the headland survives erosion and rising sea levels this will be the rock of the future.
7. Slumps and Landslides
Cliff falls are triggered by many forces of nature. Rain water seeping through the upper sandy layers becomes trapped when it reaches a layer of clay. As it lubricates the slippery clay, sections of cliff break free and drop to the beach. Ice and salt crystals from sea spray and the sea undermining the cliff base also play a part in this process.
8. At the mercy of the elements
The soft sands and clays are easily attacked by wind and rain. Wind blowing across the cliff face creates horizontal grooves. Rain water seeping down through the sand makes vertical rills and gullies.