Louise muttered an unladylike curse. ‘Bugger it! Another stone, I expect.’

Geraint gave a couple of futile wiggles to the upper part of the tube, but further pressure made no impression on the penetration.

‘It’s no good, we’ll have to haul it out and try again a couple of feet away,’ ordered his senior companion. The crossbar was now at waist height and the student hauled it upwards, then grasped the tube and pulled the rest of it out of the soggy ground.

‘D’you want to keep the core that’s in it?’ he asked and got a scathing look in response.

‘No, of course not! Every sample has to be from the same depth range. Push the damned thing out!’

With hands on hips, she watched as her slave laid the tube on the ground and, with a narrower tube with a blanked-off end, forced out a cylinder of black peat, which had the consistency of a Christmas pudding. Except that, unexpectedly, the bottom two inches of the core was almost white, instead of black.

‘What the devil’s that?’ he asked, crouching down for a closer look.

Louise did the same, then reached to her side to take a small metal spatula from the haversack. Prodding the core with it, she separated the white material from the peat and rolled it on to the back of a notebook.

‘This looks like some sort of animal material, not vegetation,’ she announced.

‘Maybe we’ve speared a dead sheep,’ volunteered the student. ‘There are plenty of those about here and some must die and end up in the bog.’

Louise peered more closely at the greyish-white cylinder, half the length of her little finger and about the same thickness. ‘It seems to be some sort of fatty wax,’ she declared.

Geraint shrugged and began to get to his feet.

‘Some sort of long-dead animal,’ he said dismissively. ‘We’d better get on and finish these holes. I could do with my lunch.’