All this growth had been aided by huge deposits of sodium nitrate that had been lying in Candor and Ophir canyons— great white bench terraces made of extremely water-soluble caliche blanco. These mineral deposits were now melting over the canyon floors and running down the streams, providing the new soils with lots of nitrogen. Unfortunately some of the biggest nitrate deposits were being buried under landslides— the water that was dissolving the sodium nitrate was also hydrating the canyon walls, destabilizing them in a radical acceleration of the mass wasting that went on all the time. No one went near the foot of the canyon walls anymore, the fliers said: too dangerous. And as they soared around in their blimpgliders, Nirgal saw the scars of landslides everywhere. Several high talus plant slopes had been buried, and wall-fixing methods were one of the many topics of conversation in the mesa evenings, after the omegandorph got into the blood; in fact there was little they could do. If chunks of a ten-thousand-foot-high wall of rock wanted to give way, nothing was going to stop them. So from time to time, about once a week or so, everyone on Shining Mesa would feel the ground quiver, watch the tent shimmer, and hear in the pit of the stomach the low rumble of a collapse. Often it was possible to spot the slide, rolling across the canyon floor ahead of a sienna billow of dust. Fliers in the air nearby would come back shaken and silent, or voluble with tales of being slapped across the sky by earsplitting roars. One day Nirgal was about halfway down to the floor when he felt one himself: it was like a sonic boom that went on for many seconds, the air quivering like a gel. Then, just as suddenly as it had begun, it was over.