Portsdown is a 69.1-hectare (171-acre) biological Site of Special Scientific Interest on Portsdown Hill, on the northern outskirts of Portsmouth in Hampshire.

[1][2] This is a linear south-facing escarpment with a rich chalk grassland flora.

The diverse insect fauna includes all the chalk downland butterflies and a population of the largest British bush cricket, Tettigonia viridissima.

On the lower slopes, raised beaches indicate former sea levels.

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