question:  What causes the shaking of the mountain?
context: WHEN THE MOUNTAIN SHOOK By Robert Abernathy Illustrated by Kelly Freas [Transcriber Note: This etext was produced from IF Worlds of Science Fiction March 1954. But its outward perils could not compare with the Ryzgas themselves, who slept within, ready to wake and conquer.... While she waited he spun a dream, attaching it to the crags that loomed over the pass, and to the frozen ground underfoot. It was black night, as it would really be when Groz and his henchmen reached this place; lurid fire spewed from the Ryzga mountain, and strange lights dipped above it; and for good measure there was an avalanche in the dream, and hideous beasts rushed snapping and ravening from the crevices of the rock. Now they were descending the steeper face of the sierra, and the way led over bottomless crevasses, sheer drops and sheer ascents, sheets of traitorous glare ice. The light shone watery and dim from beneath the hoary back of the glacier, and as they came nearer they saw why: the cave entrance was sealed by a sheet of ice, a frozen waterfall that fell motionless from the rocks above. Var eyed the ice-curtain closely to make sure that it was real, not dream-stuff; then he struck it boldly with his fist. Var saw with some surprise that all the walls were sheathed in ice—warm to the touch, bound fast against melting by the Watcher's will. Behind them began a tinkling of falling water, thawed from the glacial ridges above to descend sheet-wise over the cave mouth, freezing as it fell into lengthening icicles. Hot food and drink were before them almost at once. They ruled the Earth for four generations, and the Earth was too little for them. "Because of them we must build with dreams instead of iron, and our only fire is that of the Sun, and even now, two thousand years later, the Earth is still slowly recovering from the pangs and poison of that age.
output:  Machines
options:  The awakening of the Ryzgas  Machines  Earthquakes and tremors  Dreams spun by the Ryzgas