In parts of northern Canada and Europe great 

 stretches of barren rock lie exposed. These 

 areas were scraped clean by the massive ice 

 sheets that advanced southward during the 

 last ice age. As the ice melted, thousands of 

 lakes were formed in the rock hollows. 



sea-bed sediments, we cannot be certain of the history of the oceans. 

 Such a core sample down to the rock bed of the sediments is the 

 oceanographer's dream, and we seem to be on the verge of realizing 

 it. In March 1961, oceanographers working on the Mohole Project 

 (an attempt to drill a hole right through the Earth's crust to the 

 mantle) at Guadalupe Island went through two miles of water, 

 500 feet of sediments, and fifty feet of rock. More core samples of 

 such great depths would add enormously to our knowledge of the 

 oceans' history. 



From time to time throughout geological history great quan- 

 tities of the Earth's water have been locked up as ice during ice 

 ages. We are, in fact, today Uving in the dying grips of an ice age 

 which reached its fourth chmax about 10,000 years ago, and which 

 began between 500,000 and a million years ago. About one-tenth 

 of the total land surface is glaciated today. 



While the continents move up and down on a time scale rep- 

 resented by tens to hundreds of millions of years, invasions and 

 retreats of ice over large parts of the land may take place over 

 periods of tens of thousands of years. Although there is no general 

 agreement about the mechanisms that touch off an ice age, we do 

 not have to take them on trust, as we do wandering continents. 

 The Greenland and the Antarctic icecaps and the glaciers of the 

 Alps and other mountains are there for us to examine, and we can 

 see the marks that glaciation leaves behind. These same marks - 

 rocks scarred by ice, valleys with rounded slopes, deposits of 

 material at the foot of glaciers - can be seen in ancient rocks. At , 

 least three other major periods of glaciation have been discovered, 

 ranging back to the Pre-Cambrian of 700 million years ago. 



The amoxmt of water locked up as ice during an ice age is an 

 appreciable fraction even of the enormous volume of the oceans. 

 For example, if the Greenland and Antarctic icecaps should melt 

 suddenly, the level of the sea would rise by about 300 feet - enough 

 to turn New York, London, and Paris into underwater cities. But 

 the picture is not quite so simple ; as the great weight of ice is 

 removed from the land, the land rises up, causing subsidence else- 

 where. By examining old shore Hnes we find that interglacial periods 

 have taken place in the past, during which most of the Earth's ice 

 must have melted. We can estimate the dates of these warming-up 

 periods from fossil evidence and carbon- 14 dating, which is more | 

 precise for recent times (the last 20,000 years). From 18,000 to 

 6000 years ago the seas have risen nearly 300 feet, so it is not 

 surprising that early records of man's history mention floods and 

 deluges. A sea-level rise of 300 feet today would alter all but the 1 

 steepest rising coast lines and flood many of our most concentrated 

 centers of population, simply because such a great percentage of 

 the land is relatively low. This has undoubtedly happened at least 

 once during man's period of habitation of the planet. At the end 

 of the last interglacial period, such large-scale flooding over the 

 earth must have taken place. Although the rise of sea level averaged 

 only one foot in forty years during a warming-up period, the past 

 encroachment of water over certain areas would almost certainly 

 have been in catastrophic bursts, when rain and wind and tide con- 

 spired to produce great floods. 



The present sea level has been maintained within about ten feet , 

 either way for the past 6000 years, but that does not mean that the 



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