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The chalk deposits also contain flint nodules in large quantity. There are some parts of the downland that are covered by shallow deposits of clay; these also contain flints but to a lesser extent and are known as clay with flints. The availability of flint has led to its wide use as building material throughout the Test Valley.

Chalk is also porous and permeable and therefore the soil drains easily. As the chalk is gradually dissolved by the rain it becomes alkaline and ‘hard’ due to the calcium content. The underlying chalk also acts as an aquifer or reservoir and naturally regulates the rate at which water percolates into the springs further down where the soil is not so porous and the water has to find its way back to the surface.

The springs in turn supply the river and its tributaries with water that is oxygenated, clear and full of nutrients. Significantly, the temperature of the water remains fairly constant at about 10°C. This combination of factors together with the lack of flooding (because rain water quickly disperses and does not accumulate) has also helped create the large deposits of peat found along parts of the valley floor.