a. 

 

 Q 



-100 



-120 



-140 



-20 -15 



Years before present 



(Thousands) 



Figure 6. Late Quaternary sea level curves inferred from radiocarbon dated samples along 

 the U.S. coastlines. Modified from Dillon and Oldale (1978) 



surges, and seasonal changes in sea level, all on the order of centimeters to 

 meters. Short-term variations, particularly those associated with storms, are 

 important in increasing the effective wave base, allowing erosion to take place 

 further inland. 



Over historic time scales, significant water level changes may occur. 

 Because of the complexity of this topic, it is necessary to introduce the con- 

 cepts of relative and absolute sea level. 



A relative change in water level is, by definition, a change in the elevation 

 of the sea surface relative to some local land surface. The land, the sea, or 

 both may have moved in absolute terms with respect to the earth's gravita- 

 tional center. It is exceptionally difficult to determine absolute sea level 

 changes because tide stations are located on land masses that have themselves 

 moved vertically. For example, if both land and sea are rising at the same 

 rate, a gage will indicate that relative sea level has been stable. Other clues, 

 such as beach ridges or exposed beach terraces, also merely reflect relative 

 sea level changes. 



14 



Chapter 2 Secondary Sources of Coastal Information 



