At its northern end this incredible cliff finally became less tall, and less steep, and just above 20° North it was cut by a deep broad channel, which ran east through Lunae plateau, down onto the Chryse basin. This big canyon was Kasei Vallis, one of the clearest manifestations of ancient flooding anywhere on Mars. A single glance at a satellite photo and it was obvious that a very large flood had run down Echus Chasma once upon a time, until it reached a break in its great eastern wall, perhaps a graben. The water had turned right down this valley and smashed through it with fantastic force, eroding the entrance until it was a smooth curve, slopping over the outside bank of the turn and ripping at joints in the rock until they were a complex gridwork of narrow canyons. A central ridge in the main valley had been shaped into a long lemniscate or tear-shaped island, the shape as hydrodynamic as a fishback. The inner bank of the fossil watercourse was incised by two canyons that had been mostly untouched by water, ordinary fossae that showed what the main channel had probably looked like before the flood. Two late meteor strikes on the highest part of the inner bank had completed the shaping of the terrain, leaving fresh steep craters.

From the ground, driving slowly onto the rise of the outer bank, it was a rounded elbow of a valley, with the lemniscate ridge, and the round ramparts of the craters on the rise of the inner bank, the most prominent features. It was an attractive landscape, reminiscent of the Burroughs region in its spatial majesty, the great sweep of the main channel just begging to be filled with running water, which no doubt would be a shallow braided stream, coursing over pebbles and cutting new beds and islands every week. . . .

But now it was the site for the transnationals’ security compound. The two craters on the inner bank had been tented, as had big sections of the gridwork terrain on the outer bank, and part of the main channel on both sides of the lemniscate island; but none of this work was ever shown on the video, or mentioned in the news. It was not even on the maps.

Spencer had been there since the beginning of construction, however, and his infrequent reports out had told them what the new town was for. These days almost all the people found guilty of crimes on Mars were sent out to the asteroid belt, to work off their sentences in mining ships. But there were people in the Transitional Authority who wanted a jail on Mars itself, and Kasei Vallis was it.