There is no doubt when you are on the carboniferous limestone. Crags jut out as if the rock is struggling to release itself from its turfy skin, shedding broken stones. Sheep’s fescue, rockrose, kidney vetch and many more lime-loving species form the distinctive close-knit grassland. The signature of this rock is written all over the hill.
At Goblin Combe we cross the limestone turf, heading for my favourite slope. Melted frost has touched every leaf with diamonds and pin-cushioned the anthills with rainbow spangles. And then – so suddenly – wine-dark mounds of bell heather. Lime-hating heather, among all those lime-lovers!
This is limestone heath, an ecological oxymoron: it should not exist. Plants such as heather, wood sage, goldenrod and tormentil – all found here – are calcifuges (a term derived from the Latin to flee from chalk), preferring an acidic soil. However, limestone soil is strongly alkaline, and has its own suite of species (called calcicoles). How can it be that all these plants suck together from the same earth, roots entangled, on certain limestone slopes (including parts of Dolebury Warren)?
The question puzzled early ecologists. It wasn’t until the mid-1950s, whilst investigating the Lizard in Cornwall, that two Bristol botanists, pleasingly named Coombe and Frost, finally figured out the workings of the peculiar soil that accommodates both calcifuges and calcicoles, tracing the mystery back nearly 12,000 years. With their research in mind, we stand on the exposed hill and imagine the ending of the last ice age (it is not hard). Harsh winds scoured the thawing land and the bed of what is now the Irish Sea – sea levels were much lower than today – stripping off sandy particles. Pioneer plants, colonising the exposed Mendip ridges, trapped these flying grains and over the years formed a thin, wind-blown soil (technically known as a loess). This soil is light and droughty, while the pH balances between acid and alkaline and, in a wonderful piece of ecological magic, lies within the tolerance of both groups of plants.
Today, salad burnet scrambles through bell heather, rockrose and wood sage entwine, calcicoles and calcifuges facing the icy blast together, clinging to their wind-blown soil.
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