Wareham Common: Geographical and Geological Influences

Map showing location of Wareham Common at the western reaches of Poole Harbour, Dorset

Geographical Influences;

Wareham is in the county of Dorset which lies on the southern coast of England. Its southern location means the climate is generally mild in winter but is subject to the prevailing south-westerly weather systems. Central southern England is low lying and so, despite the exposure to the Atlantic weather, is relatively sheltered and does not usually bear the full force of incoming storms in the same way as more western and northern areas of Britain are prone to. The area receives some protection from the high ground of the Purbeck Ridge to the south which seems to divert much of the worst weather eastwards toward Hampshire and the Isle of Wight.

Wareham lies at the western reaches of Poole Harbour and stands between the point where two rivers, the Frome and the Piddle (or Trent), flow into the harbour. Whilst the town stands on high ground the surrounding areas are prone to extensive flooding in wet winters as the two rivers overflow their banks onto their flood plains. 

Being coastal the area has relatively low air pollution levels, especially in winter, but is affected by saline water saturation at times as well as salt from the winds and all three of these factors have an influence on the vegetation of the area. 

The low-lying, southerly position together with water saturation in winter provides lush vegetation around the Common.

Geological Influences:

The SSSI citation describes the underlying geology of the Common but as I said, is a bit 'technical'. Geology is a specialist subject and I am no geologist but it is important because the underlying rock and soil in any given place dictate the type of vegetation that grows and the vegetation present often influences the invertebrates and, thus, the vertebrate life that lives around it.

Many, many millennia ago the land around here was under the sea and the sea bed was formed of fine sands and clay. This particular mix here is called the ‘Bagshot Beds’ as it surfaces at various points from Bagshot Heath in Surrey, through Hampshire to Bournemouth and the southern side of Poole Harbour. The beds are about 150 feet thick and tend to be acidic and so make a hostile environment for most plants but gorse and heather seem to like it hence the abundance of heathland in the Poole basin.

Wareham Common is the flood plain of the River Piddle (or Trent in more polite circles) which has its origins on the chalk downland in the northern area of Dorset. Being from a calcareous source the water tends to be alkaline. When the river floods, as it does in most winters, the alkaline silt is deposited on the acid under bed and forms a generally neutral soil.

The clay in the underlying Bagshot Beds impedes drainage and so the result is damp or wet fertile and lush grassland all year round that is ideal for grazing cattle in summer.

Habitat Contrast:

This photograph, I hope, might help illustrate the change in vegetation as the underlying soil changes from neutral and damp to acidic and dry.

To the left is the low lying area that often floods; it is neutral soil and is usually damp right through the summer and has a mainly rush and course grass sward. To the right is the higher ground which rarely floods and here there is gorse, heathers, bramble and a different range of shorter grasses. This is because the higher ground is drier and more acidic. A clear definition between two types of geological substrates is rare, the divisions are often more blurred.

The difference between the two habitat types can be seen not only in the vegetation but also in the invertebrates present. In spring, when the cuckooflower is out in the damp area you will see orange tip butterflies as cuckooflower is the food plant for their caterpillars. If you watch, they spend most of their time on the damp side. On the higher, dry ground the small white butterfly nectars on the ragwort and bramble and does not often venture onto the damp area. The same is true of the lesser marsh grasshopper which is found in the damp area whilst the long-winged conehead is more likely to be in the dry area. All this despite the fact that the two habitats are within yards of each other.

Everything is connected, everything is important, everything has its place!