Historical Geology Understanding Our Planet\'s Past

Some 10 million cubic miles of water were tied up in the continental ice

sheets. They covered about one-third of the land surface, with an ice volume

three times greater than its present size.The accumulated ice dropped the level

of the ocean about 400 feet, advancing the shoreline tens of miles seaward.The

drop in sea level exposed land bridges and linked continents, spurring the

Figure 192Miocene-age
diatoms from the
Choptank Formation,
Calvert County,
Maryland.
(Photo by G.W. Andrews,
courtesy USGS)

Historical Geology