Coastal Morphology and the Recent Rise in Sea Level
135
depth - depending on the clearness of the water. Sessile plants, such as the geologically important calcareous algae and algal mats, generally occur no deeper than 100
m or so. Animals living in symbiosis with algae also indicate shallow water. These
animals include large foraminifera, stone corals, even certain mollusks (Sect. 6.1.3).
5.3 Coastal Morphology and the Recent Rise in Sea Level
5.3.1 General Effects of Recent Sea Level Rise. The coastal regions of uplifted
margins are largely shaped by the interplay between tectonic forces and marine
processes, especially wave erosion. Raised marine terraces are a characteristic morphologic feature of such uplifted margings; the West Coast delivers many fine
examples. In contrast, the coastal morphology of slowly sinking margins is entirely
dominated by the processes associated with the sea level itself. In North America, the
Gulf of Mexico and the East Coast provide prime examples of this type of morphology, in Europe the "Wadden Sea" mentioned earlier.
In order to understand the coastal landscape, we must be constantly aware of one
central fact: the recent rapid rise in sea level which began about 15 000 years ago
and lasted till about 7000 years ago (Fig. 5.6). The sea level rose in response to the
melting of glacial ice, chiefly the Laurentian and the Scandinavian Ice Sheets (Antarctic melting is thought to have contributed a comparatively minor amount). The
maximum addition of water to the ocean occurred some time between 14 000 and
9000 years ago. The sea level changed by approximately 120-130 m on the whole;
that is, it rose from the average depth of the shelf edge to the present positions.
As mentioned earlier, sea level is rising right now, after a long period of stability
during the Holocene. During the last few decades it has risen by some 1-2 mm per
year, presumably as the result of thermal expansion of the upper ocean layers, due to
a general warming since the tum of the centul)' (Figs. 5.22 and 7.16).
The effects of the deglacial transgression on coastal morphology and sedimentation were varied and profound. On gently sloping shores the sea advanced vel)'
rapidly. In the upper Persian Gulf, for example, the coastline must have retreated by
some 100 m per year, during the maximum rise of sea level. Whoever inhabited the
Shatt-al-Arab at that time must have been very aware of the invasion of sea.
In temperate humid regions, coastal peat bogs grew upward with the rising
groundwater level; finally they became flooded by salt water and covered by
marine sediments. Dunes were eroded by the approaching surf, except where
cemented by lime derived from shell material. Resistant matter left from the erosional
process collected as a transgression conglomerate below the march of the waves, the
basal conglomerate typical of many marine transgression sequences in the geologic
record.
The exact time sequence of the rise of sea level during the melting of the northern
glaciers has long been a matter of controversy, as the various interpretations in
Fig. 5.6a indicate. Was it an even, rapid change? Did it occur in pulses, as indicated
by terraces in many shelves and coral islands?
135
depth - depending on the clearness of the water. Sessile plants, such as the geologically important calcareous algae and algal mats, generally occur no deeper than 100
m or so. Animals living in symbiosis with algae also indicate shallow water. These
animals include large foraminifera, stone corals, even certain mollusks (Sect. 6.1.3).
5.3 Coastal Morphology and the Recent Rise in Sea Level
5.3.1 General Effects of Recent Sea Level Rise. The coastal regions of uplifted
margins are largely shaped by the interplay between tectonic forces and marine
processes, especially wave erosion. Raised marine terraces are a characteristic morphologic feature of such uplifted margings; the West Coast delivers many fine
examples. In contrast, the coastal morphology of slowly sinking margins is entirely
dominated by the processes associated with the sea level itself. In North America, the
Gulf of Mexico and the East Coast provide prime examples of this type of morphology, in Europe the "Wadden Sea" mentioned earlier.
In order to understand the coastal landscape, we must be constantly aware of one
central fact: the recent rapid rise in sea level which began about 15 000 years ago
and lasted till about 7000 years ago (Fig. 5.6). The sea level rose in response to the
melting of glacial ice, chiefly the Laurentian and the Scandinavian Ice Sheets (Antarctic melting is thought to have contributed a comparatively minor amount). The
maximum addition of water to the ocean occurred some time between 14 000 and
9000 years ago. The sea level changed by approximately 120-130 m on the whole;
that is, it rose from the average depth of the shelf edge to the present positions.
As mentioned earlier, sea level is rising right now, after a long period of stability
during the Holocene. During the last few decades it has risen by some 1-2 mm per
year, presumably as the result of thermal expansion of the upper ocean layers, due to
a general warming since the tum of the centul)' (Figs. 5.22 and 7.16).
The effects of the deglacial transgression on coastal morphology and sedimentation were varied and profound. On gently sloping shores the sea advanced vel)'
rapidly. In the upper Persian Gulf, for example, the coastline must have retreated by
some 100 m per year, during the maximum rise of sea level. Whoever inhabited the
Shatt-al-Arab at that time must have been very aware of the invasion of sea.
In temperate humid regions, coastal peat bogs grew upward with the rising
groundwater level; finally they became flooded by salt water and covered by
marine sediments. Dunes were eroded by the approaching surf, except where
cemented by lime derived from shell material. Resistant matter left from the erosional
process collected as a transgression conglomerate below the march of the waves, the
basal conglomerate typical of many marine transgression sequences in the geologic
record.
The exact time sequence of the rise of sea level during the melting of the northern
glaciers has long been a matter of controversy, as the various interpretations in
Fig. 5.6a indicate. Was it an even, rapid change? Did it occur in pulses, as indicated
by terraces in many shelves and coral islands?