Courtesy of the institute of creation research, this article succinctly makes it point so well I wanted to share it:
From Where Did the Water Come?
December 30, 2009
"And the waters prevailed exceedingly upon the earth; and all the high hills, that were under the whole heaven, were covered." (Genesis 7:19)
Lack of an answer to this question led many theologians in the early 1800s to abandon the biblical doctrine of the global Flood in favor of uniformitarianism. At the time, scientists were saying that the concept of a young earth had been disproved and that since the mountains had been around since before the time of Noah, obviously they couldn't have been covered by the Flood waters. There is simply not enough water to cover the present mountain ranges, they said. Theologians responded by proposing a local flood incapable of laying down the fossilbearing rock of the world. Even today, most evangelical theologians, denominations, and seminaries teach this compromise.
The world before the Flood was quite different from the world today. A global water vapor canopy encircled the earth and contained vast amounts of water vapor (Genesis 1:6-8). Furthermore, the topography was much less pronounced, since all present mountain ranges are made up of sedimentary rocks or volcanoes attributable to the Flood. Since it didn't rain before the Flood (Genesis 2:5), yet rivers flowed (v. 10), there must have been great subterranean reservoirs of water.
At the proper time, these "fountains of the great deep" (Genesis 7:11) spewed out their contents and the "windows of heaven were opened" as the canopy was precipitated. The breaking up of those "fountains" which were on the sea bottom implies great tsunamis elevating water to an abnormal level on land. Coupling these mechanisms with the fact that most of the earth (70 percent) is still covered with water in sufficient quantity to cover the entire earth (if it were smoothed out) to a depth of about 7,500 feet, we can conclude that the biblical story is, indeed, quite reasonable. JDM
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