He allowed himself a moment of exhilaration. Hot shit, he thought; this is the way to do a field trip.

But now the ground was getting steeper, the regolith layer thinner; the sprays raised by his footfalls were diminishing.

He tried to slow. But he’d underestimated his momentum; old Sir Isaac was pushing at his back, and the surface under his feet was slithery. He leaned back and tried to dig his heels in, but that succeeded only in tipping him over backwards.

He slid six, ten feet on his butt, deeper into the rille. In his stiff Shuttle suit it was like sliding down a ski slope in a box.

He tumbled to a halt.

He sat there in darkness, breathing hard, his torch showing him only a few yards ahead, so close was the rille’s horizon now.

His rope gathered itself up, went taut, and he got a good hard tug on the chest. Geena, telling him to behave himself.

Well, she was right. It was going to be a lot harder coming back up.

He got up and slapped ineffectually at his thighs, trying to dust himself off. He held onto the rope, and stepped cautiously, ever deeper, his feet scraping now over almost bare basalt.

He came to a place where the slope was steeper than one in one. There was a basalt ledge here, a place where one of the lava strata had become exposed; it made a place to stand. When he looked up, his torch showed him the rille wall tipping up and away from him, dauntingly steep and tall.

He wasn’t sure he could go any further in. This would have to do.

He took his instrument bag off his shoulder, and placed it carefully at the back of the ledge. Then he got to his knees, and crept forward to the lip of the ledge. The suit fought him the whole way, but he crawled determinedly.

He stuck his head over the edge.

His torchlight splashed down over the steepening rille wall. More basalt layers. He inspected them briefly, noting their thickness, their difference in composition.

The whole history of mare volcanism was laid out here, clear as a road-map. He longed for a couple of grad students, a truck-load of sample bags. Maybe just by visual inspection, a couple of samples, he could achieve some good science here…

But he knew he mustn’t. His objective lay further on, and he must hurry to achieve it before his life support limits cut him short.

He lifted his head, and let his torch beam play further down the slope.

It looked as if the base of the rille, here, was no more than thirty or forty feet below him. The surface was smooth, silvery and flat.

It looked as if a river of some fluid had been dammed here.

Moonseed dust.