On their fourth day in the laminate terrain, the plateau walls flanking their flat wash curved together, and they drove up the cleavage onto a higher plane; and there before them on the new horizon was a white hill, a great rounded thing, like a white Ayer's Rock.  A white hill-it was ice!  A hill of ice, a hundred meters high and  a kilometer wide-and when they drove around it, they saw that it continued over the horizon to the north.  It was the tip of a glacier, perhaps a tongue of the polar cap itself.    In the other cars they were shouting, and in the noise and confusion Nadia could only hear Phyllis, crying "Water!  Water!"

Water indeed.  Though they had known it was going to be there, it was still startling in the extreme to run into a whole great white hill of it, in fact the tallest hill they had seen in the entire five thousand kilometers of their voyage.  It took them all that first day to get used to it:  they stopped the rovers, pointed, chattered, got out to have a look, took surface samples and borings,  touched it, climbed up it a ways.  Like the sand around it, the ice hill was horizontally laminated, with lines of dust about a centimeter apart.  Between the lines the ice was pocked and granular; in this atmospheric pressure it sublimed at almost all temperatures, leaving pitted, rotten side walls to a depth of a few centimeters; under that it was solid, and hard.

"This is lot of water," they all said at one point or another.  Water, on the surface of Mars. . . .

The next day the glacier hill  formed their right horizon, a wall that ran on beside them for the whole day's drive.  Then it really began to seem like a lot of water, especially as over the course of the day the wall got taller, rising to a height of about three hundred meters.  A kind of white mountain ridge, in fact, walling off their flat-bottomed valley on its east side.  And then, over the horizon to the northwest, there appeared another white hill, the top of another ridge poking over the horizon, the base remaining beneath it.   Another glacier hill, walling them in to the west, some thirty kilometers away.

So they were in Chasma Borealis, a wind-carved valley that cut north into the ice cap for some five hundred kilometers, more than half the distance to the Pole.  The chasm's floor was flat sand, hard as concrete, and often crunchy with a layer of CO2 frost.  The chasm's ice walls were tall, but not vertical; they lay back at an angle less than forty-five degrees, and like the hillsides in the laminate terrain, they were terraced, the terraces ragged with wind erosion and sublimation, the two forces that over tens of thousands of years had cut the whole length of the chasm.