Norfolk Wildlife Trust looks at Cranberry Rough lake

Norfolk Wildlife Trust’s Cranberry Rough nature reserve has a hidden secret, for lying beneath rough grassland, fen, and scrubby woodland is a lost lake, or ‘mere’ to use the exact term.

Cranberry Rough’s Hockham Mere is situated in the Brecklands, a unique UK landscape which contains twelve active meres in total.

These naturally occurring spring fed bodies of water are incredibly rare and known as fluctuating meres.

They can, rather oddly, contain more water during a hot summer than in winter.

This is due to a delayed response to groundwater levels, with the quantity of rainfall from previous years dictating the amount of water that percolates up through the porous chalk.

These exceptional geological features have remained intact since the end of the last Ice Age.

The largest, at seven hectares, is Fowl Mere, but the now vanished Hockham Mere is estimated to have been a whopping eighty-one hectares, with core samples suggesting it may have been up to thirty feet deep.

Like the other Breckland meres, it formed over 10,000 years ago, probably by the dissolution and collapse of the underlying chalk bedrock.

Boreholes have shown that the lowest layer of sediment in Hockham Mere is sand, which would have been washed or blown in from the bare land left behind as the last glaciers retreated.

Since then, the basin has filled with thick silt, and a layer on top of more recently formed peat.

The gradual build-up of sediment has provided scientists with a pollen record that gives an insight into the development of vegetation over thousands of years.

Initially patchy areas of birch and Scots pine surrounded the lake, followed by thick forests of hazel, oak, alder and elm as the climate warmed.

The mere’s ancient mud contains seeds and organic matter that indicate it would have been rich in aquatic plants. It was dominated by the exceptionally rare holly-leaved naiad, a relic of post-ice age Britain, and now only found at a few locations in the Norfolk Broads.

Evidence has been found of people settling around the original shoreline as far back as the Mesolithic era, and several flint tools have been discovered.

Remains of charcoal suggests campfires or local attempts at forest burning.

Radiocarbon dating and pollen samples show a significant rise in grass species and suggests that clearings in the surrounding forest had certainly been made by Neolithic farmers 5,000 years ago.

In more recent times written accounts infer that it was still an open lake in the Tudor period, although the mere had already started to silt up by the time it was first drained in the 17th century.

However, drainage requires a lot of upkeep, and the area was regarded as ‘very wet and swampy’ in the 1920s, being well known to local naturalists for its ‘noteworthy flora’.

The Forestry Commission commenced drainage again in the 1930s but had, fortunately, abandoned this work by the 1960s.

Now, as an NWT nature reserve, the site has returned to being an important wetland site.

The stabilised water levels and lack of pollution has ensured that it maintains an exceptional range of plants and invertebrates, many unusual for East Anglia, with some being rare for the UK.

Much of the lake basin contains layers of peat that has formed into an acidic bog.

Cranberry, only found at three sites in Norfolk, and royal fern, rare outside the Broads, grows here, as does bog-bean, marsh cinquefoil and cowbane.

The narrow small reed grass, another relic from the Ice Age, has only one other record in Norfolk at NWT Thompson Common.

More than 60 species of spider have been recorded, along with many rare beetles, numerous bird species have bred, including the threatened and declining willow tit.

Following previous clearance of dense willow scrub, snipe returned to breed, and it’s one of the few places that their strange ‘drumming’ display can be heard.

In autumn good numbers of them arrive to spend the winter on the site, having migrated from Eastern Europe and Russia.

However, this open fen habitat is at risk of being lost to the encroaching alder carr woodland, so NWT staff have engaged in a programme of scrub clearance to maintain and restore this important and unusual Norfolk habitat.

Due to the site’s challenging terrain this work has proved difficult, and without care, potentially dangerous.

At the heart of the vanished mere the vegetation has grown across open water much like a skin on custard.

Referred to as hover or quaking bog, walking across it gives a sensation of the ground moving in bouncing waves, and if you tread where the hover is thin you can fall through, one’s feet rarely touches a solid bottom.

To achieve this habitat restoration work NWT staff employed floating fire trays to burn the cut material, aluminium liggers to spread their weight across the hover and a military style amphibious tracked vehicle.

This work can only be carried out in autumn, of course this often means the site is even wetter, however this avoids disturbance to breeding birds and damage to flowering plants.

Despite these conditions, this important and tough work has ensured that a unique habitat remains in pristine condition.

Cranberry Rough lies within the Forestry Commissions Thetford Forest. The Great Eastern Pingo Trail passes down the western edge of Cranberry Rough along a disused railway line. This provides ideal vantage points to view this spectacular site, which would otherwise be inaccessible due to the treacherous ground conditions.