Nature Studies by Michael McCarthy: Chalk – the great giver of wildlife richness

I have a great fondness for maps. I buy a map whenever I travel anywhere and I keep it, and I can browse through maps like you can browse through magazines, letting the imagination wander over this river and that wood, or this village and that lane; but my favourite map doesn't have rivers or woods marked on it, or villages or lanes, although, very faintly, it does have major towns.

It is "the 10-mile map" – the British Geological Survey's geological map of Britain, at the scale of 10 miles to the inch, which shows the country not in terms of administrative regions, or landscape features, but in terms of the underlying rocks. The various strata are variously coloured, for differentiation rather than resemblance (although the dull orange marking the Triassic sandstone of the Wirral peninsula where I grew up is remarkably similar to the stone itself), and what excites me whenever I unfold it is the great brilliant band of emerald green splashed diagonally across England from bottom left to top right, from Dorset to the East Riding of Yorkshire.

It is the chalk. The green on the map represents the soft white rock of the chalk hills, from the Dorset Downs up through the Chilterns to the Lincolnshire Wolds and beyond, formed from the remains of trillions of tiny organisms which filled the seas when the dinosaurs ruled on land, and settled on the seabed when they died.

It is pure calcium carbonate. It's not often celebrated. Perhaps that is because chalk as a substance may seem familiar and banal; it is for writing on blackboards or coating the ends of billiard cues. Yet chalk in the landscape is anything but; it is the great giver of beauty and wildlife richness. The chalk hills, often referred to as downs or downland, are what is most typical of the loveliness of the southern English countryside, as their form is gentle and flowing (like the contours of the human body, it has been said), quite unlike the paternalistic craggy dominance of the mountains of Wales and Scotland.

Even more, they host the best of our biodiversity. The flora of the chalk is stupendous, not just for the flower carpet of the short-cropped downland filled with scented wild thyme and horseshoe vetch and milkwort, and the chalk grassland flowers on Salisbury Plain such as wild marjoram and rock rose, but also for the rarities: a large number of our most uncommon and most spectacular orchids grow on the chalk, from the military orchid (once thought extinct in Britain) and the monkey orchid, to what may now be our most endangered wild flower, the red helleborine, blooming in a few secret places in the shadows of the Chiltern beechwoods.

The insects of the chalk landscapes are no less special: a wonderful assemblage of bumble-bees, and a heart-stopping panoply of butterflies in summer like the dark green fritillary and the silver-spotted skipper, and of course the blues, the Adonis blue, the chalkhill blue, and the common blue, all closely-related variations on a theme of brilliance. Even the birdlife of the chalk landscapes is exceptional, from stone curlews to skylarks: there are 14,000 pairs of skylarks on Salisbury Plain – even today – and in spring they pour down a shower of song which seems as much a part of the air as the wind.

But the rivers of the chalk are what I love best. The chalk streams, as anglers have named them, one word instead of two, are the most beautiful rivers anywhere on the planet. There are about 60 of them, from small rivulets to the queen of them all, the River Test in Hampshire, and they are characterised by the clearest and purest water in the natural environment, often referred to as "gin-clear". (The chalk filters it, and water companies are desperate to get their hands on it).

The chalk streams are immediately arresting. Their flow is stately, never sluggish, never torrential (the Test is like the Loire in miniature) and they are filled with a profusion of fish, of aquatic wild flowers like the ranunculus, the white water buttercup, and of aquatic insects of which the most magnificent is the mayfly, which is emerging just about now. Over the next month there will be clouds of mayflies over the chalk streams, the males dancing to attract females in one of Britain's most stunning wildlife spectacles, before mating and falling back to the water surface where trout will greedily gobble them.

Even a glance at a chalk stream lifts my spirits, and every time I catch sight of one I feel like offering up a prayer in praise of calcium carbonate. To some people it might be merely the stuff they use to powder the lines on tennis courts, but it also gives us the gentleness of the English landscape, the pyramidal orchid, the Adonis blue, and rivers which flow straight into your heart. And when I unfold the 10-mile map and look at the great green splash on it, that's what I see.

Lovely lilacs

Sorry to be banging on about blossom, after having written about the stuff for the last two weeks, but I have to put in a word for lilac. This is a cultivated shrub rather than a part of the natural world now, of course, so really lilac comment belongs in a gardening column. But lilac this year has never seemed so spectacular. Is it the freezing winter and the late spring? I've no idea. Yet the branches of the lilac in our garden are quite literally bending under the weight of their blossoms, with that distinctive woody scent (with a hint of a sour edge to it) perfuming the air. Something else to give thanks for.

m.mccarthy@independent.co.uk

For further reading

'Wild Flowers of Chalk and Limestone' by JE Lousley (Collins New Naturalists series, no.16)