With the coming of the Atlantic-the Mesozoic split-the principal drainages of the American East at first continued to flow toward the Midwest. A part of the plate-tectonic story is that a great deal of heat accompanies tectonic rifting and the heat lifts the two sides of the rift like trapdoors facing each other. The shores of the Red Sea look like that. On both sides are mountains, nine, ten, twelve thousand feet high. Extremely short steep rivers fall into the Red Sea. Principal drainages-the intermittent rivers of Arabia-run eastward almost from the east shore many hundreds of miles, and from near the west shore Egyptian rivers run west to the Nile. The world’s mid-ocean ridges-the spreading centers of plate tectonics -are configured like the rift of the Red Sea. Typically, the two sides are of gentle pitch, and gradually rise six thousand conference room eindhoven feet higher than the flanking abyssal plains. Groovelike down the ridgelines run submarine rift valleys. Into the rift valleys of eastern Africa pour extremely short steep rivers, while long ones, like the Congo, rise close to the rift but flow away westward a thousand miles to the sea. It was the discovery and confirmation of spreading centers that opened the story of plate tectonics-and this is still the aspect of the theory that provokes the least debate. Eastern America, in Jurassic time, gradually subsided. The conference room rotterdam present explanation would be that as the ocean grew wider and the heat of the spreading center became more distant, the region cooled like a collapsing souffle, while the weight of water and accumulating sediments also pressed down on the continental shelf. In any case, the broad package of land that had tilted northwestward for approximately three hundred million years now seesawed and began again to tilt the other way. Rivers turned around, pooled temporarily against the ribs of the washboard, and ran over them, seeking weaknesses in the rock. Anew, the running water began to etch out the country. It was a process analogous to photoengraving, wherein acid differentially eats pictures into treated sheets of metal. The new and reversed eastern rivers differentially eroded the Appalachian structures.