Friday, 22 February 2008

Lime kilns, mines and souterrains

During the recent spell of unseasonably pleasant weather (back to normal now - there's rain going sideways past my window) I and a bat group colleague spent an interesting afternoon in an upland area of Midlothian, looking at various man-made structures with potential to be either hibernacula or summer roosts.

Information on the sites had come from a mixture of word of mouth and references on the CANMORE archeological database (see "Armchair roost-searching"). Some CANMORE references can be little more than a name and grid reference, others include detailed archeological descriptions, which can be fascinating.

The first site was a limestone mine. This is a known hibernaculum, which has been grilled in the past, partly to protect hibernating bats from disturbance and partly to prevent adventurous youngsters from getting themselves into difficulties. I wanted to take a look, as I hadn't previously seen the site and it apparently used to have an underground link with another site, which Lothians Bat Group survey every winter for hibernating bats.

Hibernaculum grilles have quite widely-spaced, horizontal bars, to prevent human access, but make it easy for bat to fly through. It seemed some local wit had been there before us, and left his thoughts for all to see.....

The next plan was to look at several lime-kilns, which are prolific in the limestone areas of Midlothian. These are large stone-built structures, usually built into a hillside. Inside is a tall, brick-lined charging-column, into which limestone and fuel was placed. At the base are draw-holes, usually with a barrel-vaulted roof, which are used to control the air-flow, and to draw the completed lime out, ready to be mixed with water and used as fertiliser or building mortar.

Barrel-vaulting can lose it's mortar over time, creating deep crevices, perfect for hibernating bats. Also, the stonework of the kiln can decay, creating roosting opportunities. Unfortunately, none of the three kilns looked at had many crevices in the draw-holes, but all had decaying stone-work to a greater or lesser extent.

One of the kilns was a massive structure, with two charging columns and six draw-holes, all in excellent condition, making me wonder if it may have been rebuilt in the 1930s, when the government offered incentives to farmers to burn lime, due to concerns about deteriorating land fertility.

At the other end of the scale was a badly-decayed kiln within dense woodland. Landslips had partially buried it and we found a solitary bat dropping in a huge crack in the stonework. A single dropping doesn't make a roost, but it may be worth a return visit in the summer.

The third site had a lovely Barn owl (Tyto alba) roost: a deep hole, high up on the side of the kiln, with long white streaks, left by the owl's characteristic runny droppings. Inside and on the ground below were plenty of large, dark-coloured owl pellets.

There was no sign of the owl (or owls), though it could have been deep inside it's roost-hole, out of sight. Not a bat roost, but it was very nice to see signs of this scarce bird, all the same.

Having seen enough lime kilns to last a while, the last site of the day was a soutterain. These are iron age (about 700BC-500AD) structures, believed to be underground storage facilities. In an age before refridgeration, building a shallow, stone-lined tunnel was a good way to keep food dry and cool, to last through the winter months.

This particular soutterain lies in a small, fenced-off area in the centre of a field. Accessed via a low roofed doorway (and when I say low, I'm talking about crawling on hands and knees), and short entrance tunnel, it is almost 16 metres long and up to 2m wide and high. The walls are made of unmortared stone, full of very deep crevices.

No hibernating bats were visible, but there could have been legions of them, out of sight! With a low, stable temperature and high humidity (yes, it's mud you have to crawl through...), the conditions are ideal for a hibernaculum, especially as it is in area full of excellent bat habitat.

The soutterain is out of sight of the nearest road and probably not known to many people, so is unlikely to suffer much disturbance, especially in winter. That said, a team of Powergen workers, dangling from nearby power lines looked fascinated by what we were up to.

More information about barn owls:

No comments: