Geologists are aware that the level of the ocean surface is not fixed, but instead a dynamic feature of our planet. Sea-level during the Last Glacial Maximum (LGM, 20,000 years ago) was 125 - 130 m lower than it is at the present time. The image below shows the modern margin of the Gulf of Mexico.

<a onClick="window.open('/olcweb/cgi/pluginpop.cgi?it=jpg:: ::/sites/dl/free/0072402466/30425/VirtualVista1401.jpg','popWin', 'width=NaN,height=NaN,resizable,scrollbars');" href="#"><img valign="absmiddle" height="16" width="16" border="0" src="/olcweb/styles/shared/linkicons/image.gif"> (123.0K)</a>

Using digital elevation data, geologists can develop maps showing the continental margins during episodes of lowered sea-level. The image below shows the margin of the Gulf of Mexico and other parts of the western Atlantic (a portion of Cuba and the Bahamas) as they would have appeared 20,000 years ago during the sea-level lowstand. Note that large areas of the continental shelf and the Bahamas platforms are exposed well above sea-level.

<a onClick="window.open('/olcweb/cgi/pluginpop.cgi?it=jpg:: ::/sites/dl/free/0072402466/30425/VirtualVista1402.jpg','popWin', 'width=NaN,height=NaN,resizable,scrollbars');" href="#"><img valign="absmiddle" height="16" width="16" border="0" src="/olcweb/styles/shared/linkicons/image.gif"> (133.0K)</a>

Geologists can also model sea-level rise using the same types of digital data. In the view below, sea-level is 15 m higher than at present. Note the dramatic change in the outline of the Gulf of Mexico as much of south Florida is inundated. Also note that the Bahamas are completely submerged, and a narrow strip of land is all that remains of Cuba.

<a onClick="window.open('/olcweb/cgi/pluginpop.cgi?it=jpg:: ::/sites/dl/free/0072402466/30425/VirtualVista1403.jpg','popWin', 'width=NaN,height=NaN,resizable,scrollbars');" href="#"><img valign="absmiddle" height="16" width="16" border="0" src="/olcweb/styles/shared/linkicons/image.gif"> (117.0K)</a>

To learn more about sea-level changes and the geologic processes that shape coastal areas, read chapter 14 of your textbook.