Kidney Vetch | ObsessedByNature

GT LWT group setting off into the reserve

Our entire LWT volunteers group (well, everyone who wanted) took a day off work and travelled the tube, overground, and tram to get to New Addington. From the end of the tram line, we walked a few steps to Hutchinson’s Bank.

This is the start of the group of beautiful and quiet LWT reserves that line this part of the warm south-facing scarp slope of the chalk of the North Downs.

The chalk formed as the tiny shells of marine algae (coccolithophores) rained down on to the seabed of a warm shallow sea during the Cretaceous period. The upper chalk here formed 100 to 66 million years ago. It then hardened, and was eventually crumpled by tectonic movements to form a ridge above what is now the Weald of Kent and Sussex. That ridge eroded away, leaving a north-facing scarp on the South Downs, and the south-facing scarp of the North Downs that we were standing on.

The chalk is a soft limestone, rich in calcium, which makes the soil on it basic (alkaline), unlike the neutral or acidic London Clay and Thames Flood Gravels. The resulting soil is a thin, free-draining Rendzina on steep slopes, and a thicker Brown Earth in the valleys. These support a wealth of flowers such as Marjoram, Scabious, and several Orchids that like an alkaline soil. Those in turn support many species of butterfly; Andy says there are 40 species on the local list.

We strolled eastwards through the woods and chalk grassland of Hutchinson’s Bank, down the slope and through the wood of Three Corner Grove, and across the road to the third reserve, Chapel Bank.

The area is a few miles west, but on the same chalky slopes that Darwin visited from Down House: Down Bank and the little hill he called ‘Orchis Bank’ where he found specimens, and most famously described as a “tangled bank” in his 1859 book The Origin of Species.

Oh, all right then, here’s his description. You can decide for yourself if the chalk hills of the North Downs led him to devise his theory of evolution by natural selection:

It is interesting to contemplate a tangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent on each other in so complex a manner, have all been produced by laws acting around us.

I gave a brief introduction to the geology, soils, and ecology; Andy did the same for the butterfly fauna.

To see the images uncropped and at full size, open them in a new tab or window.

There was great excitement among the butterfly enthusiasts in the group when a large orange-brown butterfly flew past with the strong purposeful flight of a Fritillary. Two species, the Silver-Washed and the Dark Green, are native to the area; they are larger than the introduced or “escaped” species, the Glanville, Heath, and Marsh Fritillaries which can also be seen here.

Eventually, amidst much stalking of the first specimen, which proved to be a Silver-Washed, three more of that species came by, chasing each other about, and then obligingly landing on some of the abundant Marjoram which was aromatically in flower and nectar among the grass.

Here’s Andy’s photo of one of them.

Silver-Washed Fritillary on Marjoram, taken by Andy

On Therfield Heath SSSI (Royston Hill) with Yellow Rattle, with the plains of Cambridgeshire behind

Much of East Anglia is flat, and very low-lying, indeed parts of the Fens are basically at sea level. But there are some hills, and even a Chalk escarpment. It’s pretty low, but still affords a fine view northwards across the plains. The nearly complete “failure of a major escarpment” is the result of the Ice Ages – the ice sheet, maybe a mile thick, ground interminably over the hills and plains, reducing most of the chalk to rock flour with flints, creating the sticky Boulder Clay that carpets much of eastern England. But at Royston, a delightful range of low hills survives, and has somehow survived the plough and the developers.

The grass of Therfield Heath (Royston Hill) is thinned by the parasitic Yellow Rattle (Orobanchaceae, the Broomrape family of parasitic plants): it helpfully weakens the grass, allowing in many other flowers, so it’s a bit of a Keystone Species, one on which the health of the ecosystem depends.

A colourful assemblage: Yellow Rattle – Red Clover – Birdsfoot Trefoil

The plants let in by the weakening of the grass include a colourful and increasingly rare assemblage, which includes Kidney Vetch, Birdsfoot Trefoil, Rockrose, Thyme, Wild Mignonette and many others.

Rockrose and Thyme, attractive plants of Chalk Grassland

The flowers in turn support butterflies including Marbled White, Meadow Brown, and Small Heath. Half-a-dozen Skylarks were singing all around; one got up pretty close to us for a brief song-flight, quickly followed by several of his neighbours. A Swift dashed overhead. All these once-familiar and widespread species are becoming rather special, a measure of the ecological disaster that has spread not just across England but across Europe and, really, the whole world.

Therfield Heath landscape with Elder-Hawthorn bush

Perforate St John’s Wort with interesting small pollinators

It’s interesting to see a pattern in the distribution of plants. I last saw Dropwort on Helsington Barrows, a limestone hill at the southern edge of the Lake District (not a place with much limestone, given the area’s ancient volcanic rocks and slates). Here it’s on a very different form of limestone, chalk, but if the soil is alkaline and supports open grassland, that’s fine with Dropwort. It’s a plant with a beautiful foamy white cluster of flowers on a rather isolated stalk rising from the grassland. The attractive foaminess is reminiscent of Meadowsweet, Filipendula ulmaria, and indeed Dropwort is in the same genus: it’s Filipendula vulgaris, though it could hardly be called common these days.

ObsessedByNature

Tag Archives: Kidney Vetch

LWT Nature Walk at Hutchinson’s Bank

Therfield Heath, Royston – surviving chalk grassland in East Anglia

The English seem unemotional … except for their passion for nature