They had travelled again. That was certain. The lake was gone, so too was the setting sun and the forest valley. But it was still damp. And cold. Very cold. Osha squatted close to the ground, as much to examine the sand as it was to warm her bare calves against her thighs. There was no soil here, nor anywhere, just said, gray sand and cobbles, with patches of lichen and scrub hugging the ground. The sky above was battleship gray and around their feet seething mists scudded about, blown by sharp, biting gusts. The landscape about them seemed to fall away, hidden by clouds and haze. All around them they could hear the sound of rushing water, but they could see no water of any kind aside from still pools and shallow mud puddles.

Worse still, several hours ago, when they'd appeared at the base of a bluff, Osha's phone had no signal at all. The Doctor, still fiddling with it, assured her that they were far beyond civilization, probably by about ten thousand years, give or take a hundred thousand or two. As disturbing as that was, Osha was far more concerned about the lack of any food or shelter. The Doctor, as far as she could tell, had no need of food for some reason, nor any of the other... subsequent functions. Again, very odd. Again, also very uncomfortable when she had to excuse herself at one point behind a boulder when nature called. The Doctor politely ignored the incident.

After wandering for some time, her stomach started to growl and she began to eye his celery stalk hungrily... "Doctor, why do you wear a rather limp stalk of celery on your lapel?'

The Doctor coughed awkwardly. 'There are several reasons, but mostly, I've found that, when in doubt, when facing friend or foe, confuse them first. If nothing else it buys you time to think of something else, even if it's just following your own instinct. Wearing a vegetable, if nothing else, tends to give people a moment's pause...' He fiddled some more on the keypad, then closed the casing of her phone and tossed it back to her. 'Now that,' remarked the Doctor, sprinting ahead, 'is odd indeed. Don't you think?'

Apparently, the strange man seemed to think he'd get farther by asking her questions than waiting for her to ask them. Common ground. Equal footing. Even though she was growing more and more certain that she was in no way his equal, Osha appreciated his gesture of respect. 'I'm afraid you're going to have to be a bit more specific,' She said when she caught up with him. He seemed to be surveying the terrain ahead.

The Doctor waved his hands in the air in front of him. 'Isn't geology wonderful? I once knew a geomorphologist named Price who once said that "attempting to interpret the origins of landforms many thousands of years after their formation and after they have been subjected to modifications by weathering and mass movement it is like asking someone to work out the plot of a 1000-page detective novel from the last five pages."'

'If only your friend thought of time travelling via rain, he'd have had a solved a good deal of mystery novels...' She studied the area before her, trying to see what he was getting at. Geology had been her worst subject at school – although she understood its merits, watching mud settle hardly seemed like an exciting career option.

The Doctor seemed to be patiently waiting, so she did what she asked her students to do during field camp: she talked it through. 'There are many gently sloping hills in close proximity; the largest, flattest of these hills has a pool of water within the depression on top, with gently sloping hills around it. The pond appears to be fairly deep, with no sign of vegetation...'

'And?'

'And...'Osha pursed her lips and blew out a poof of impatient breath. 'There is an empty stream channel leading down from it... no hang on, that can't be right... the channel that's feeding into it leads up from...'

'Downhill,' the Doctor finished. 'There is a lake on top of a hill, with no apparent drainage except for a stream that feeds it into it from downhill, which would require enormous pressure, like say...' the Doctor began to trot away in the direction of the stream.

'The stream that runs into it,' Osha began to grasp what he was getting at and the excitement make her voice quaver, 'was flowing uphill! Except of course, streams don't flow uphill.' She followed him, her steps faster and faster, forgetting the cold in her excitement. He trotted past the rolling hills and stood upon a ridge.

There was a break in the fog and Osha, for the first time, could see finally see what lay before them. The sight made her felt very, very, very small.

It was far away, but it was so massive in width and breadth that it seemed to have swallowed the horizon and bit into the sky. Osha had expected it to be white, but the huge wall before them was a dark blue gray streaked with gnashing textures of brown and black. Everywhere she looked, the massive wall seemed to stretch around them, with nothing between it and them but miles and miles of outwash plains. Thousands of shallow streams snaked and roared from the front of the continental glacier and out on the plains. Sometimes the channels slid and crossed each other, while others flowed together to form massive channels that growled and bellowed with their volume and greed for ice and sediment.

'like... say,' Osha continued, 'a stream flowing beneath a glacier two miles high?'

'Indeed,' the Doctor smiled, but he stopped her when she made to move down onto the outwash plain. 'Treacherous,' he explained, 'lots of gloop.'

'Gloop?'

'Hmmm... I agree, not very scientific, but very accurate. The soil can be very saturated, it's quite possible that you can sink up to your knees... not to mention any buried ice blocks that may have melted and the empty cavities could swallow you up whole, or outburst floods the size of the Amazon... not terribly frequent, but still... Then there's the fact that it would take us several hours to get near the ice front.' He looked her up and down and appeared to notice for the first time that she was still only wearing shorts and a light shirt. 'Besides we're not exactly dressed for it.' He swept his coat off and nestled it across her shoulders. Osha pointedly looked away from the celery. She wasn't near hungry enough to start snacking on accessories. At least not yet. Perhaps for breakfast... if they made it through the night.

'Still,' he continued 'we're on Earth, at least that's something, late Pleistocene obviously.' Still snug in his sweater and long shirt, he slipped his hands into his trouser pockets and pondered the ice face. 'Do you know, I don't think I've ever spent much time in the Ice Age?'

'Too dangerous? Too dull?' Was the best Osha could come up with. The Doctor's statements were so odd most of the time she didn't really know how to take them seriously.

'Not at all... It just always seemed far, far too... silly.' The Doctor tugged his hat tighter to his brow as the stiff wind threatened to yank it off. 'Beavers nine feet long with incisors nearly two feet in length, five hundred pound rats the size of tanks... all a bit much really.' He scanned the horizon, apparently looking for any approaching giant beavers or rats that might harass them. 'Except for the sabre tooth tigers of course... silly, but still massive, terrifying hunters, respect is due... '

'Again,' Osha interjected. 'The lack of any animal life here is very striking.'

'Yes. Interesting.' The Doctor sniffed the air. 'Do you want to know something very disturbing?'

'Only if by disturbing you mean that we may have walked past a Starbucks two miles back without noticing, then yes.'

'The level of detail, the scale, genuine subglacial landforms...I think we actually are on Earth. In the Pleistocene.'

'You did just say that.'

'No I mean, we really are. I thought- I assumed that we were still in my ship, that this was all just a construct or that we were somewhere else made to look like it, but this,' he swept his hand at the landscape before him, 'this is very real... and yet the area around the bluff where we arrived was most definitely not.'

Osha tried to establish what he was saying. 'So we are on Earth in the Ice Age, but the bluff where we arrived was something else... artificial?'

'Well, overlain on the immediate surroundings, but nothing more than a copy. I imagine that the animals pick up on the discontinuity and stay away. However, I suspect that the bluff and the lake shore are one and the same thing, they've just been shifted through time and space and reconfigured to blend into the surroundings.'

'Taking us with it.'

'Mmmmm... it's not supposed to operate that way at all... must be highly unstable.'

'And you know what it is, I suppose.'

'Indeed, old technology, from my own people, however it was meant to be deactivated millennia ago, although the details are foggy. Do you know, I think I'm finally starting to understand what may have happened. We have to go back to where we arrived, and quickly before it destabilizes again.'

The howl, when it cut the air, made Osha leap backward, away, oddly, from the Doctor.

'No animals of any kind... except giant wolves?' Osha asked nervously. She couldn't say why she'd jumped away from him just then. Just instinct. Which bothered her a bit more than that terrible wolf cry she'd just heard. As if she somehow knew that it was after him...

'Yes,' the Doctor turned and began to usher her back the way they had come, 'I think it's time we left – quite quickly!'