Information on past climates is of primary relevance to
archaeology because of what it tells us about the
effects on the land and on the resources that people
needed to survive. The most crucial effect of climate
was on the sheer quantity of land available in each period,
measurable by studying ancient coastlines.
These have changed constantly through time, even in
relatively recent periods, as can be seen from the
Neolithic stone circle of Er Lannic, in Brittany, France
(once inland but now half submerged on an island)
or medieval villages in east Yorkshire, England, that
have tumbled into the sea in the last few centuries
as the North Sea gnaws its way westward and erodes
the cliffs. Conversely, silts deposited by rivers
sometimes push the sea farther back, creating new
land, as at Ephesus in western Turkey, a port on the
coast in Roman times but today some five kilometers inland.
Nevertheless, for archeologists concerned with the long
periods of time of the Paleolithic period there
are variations in coastlines of much greater magnitude
to consider. The expansion and contraction of the
continental glaciers caused huge and uneven rises and
falls in sea levels worldwide. When the ice sheets
grew, the sea level would drop as water became locked
up in the glaciers; when the ice melted, the sea
level would rise again. Falls in sea level often exposed
a number of important land bridges, such as
those linking Alaska to northeast Asia and Britain to
northwest Europe, a phenomenon with far-reaching
effects not only on human colonization of the globe but
also on the environment as a whole-the flora
and fauna of isolated or insular areas were radically
and often irreversibly affected. Between Alaska
and Asia today lies the Bering Strait, which is so
shallow that a fall in sea level of only four meters
would turn it into a land bridge. When the ice sheets
were at their greatest extent some 18,000 years
ago (the glacier maximum),it is thought that the fall
was about 120 meters, which therefore created not
merely a bridge but a vast plain,1,000 kilometers from
the north to the south, which has been called
Beringia. The existence of Beringia (and the extent to
which it could have supported human life) is one
of the crucial pieces of evidence in the continuing debate
about the likely route and date of human
colonization of the New World.
The assessment of past rises and falls in sea level
requires study of submerged land surfaces off the
coast and of raised or elevated beaches on land. Raised
beaches are remnants of former coastlines at
higher levels relative to the present shoreline and
visible, for instance, along the Californian coast
north of San Francisco. The height of a raised beach
above the present shoreline, however, does not
generally give a straightforward indication of the
height of a former sea level. In the majority of
cases, the beaches lie at a higher level because the
land has been raised up through isostatic uplift
or tectonic movement. Isostatic uplift of the land
occurs when the weight of ice is removed as
temperatures rise, as at the end of an ice age; it
has affected coastlines, for example, in Scandinavia,
Scotland, Alaska, and Newfoundland during the
postglacial period. Tectonic movements involve
displacements in the plates that make up Earth’s crust.
Middle and Late Pleistocene raised beaches in
the Mediterranean are one instance of such movements.
Raised beaches often consist of areas of sand, pebbles,
or dunes, sometimes containing seashells or
piles of debris comprising shells and bones of marine
animals used by humans. In Tokyo Bay, for example,
shell mounds of the Jomon period (about 10,000 to 300 B.C.E.)
mark the position of the shoreline at a
time of maximum inundation by the sea (6,500-5,500 years ago),
when, through tectonic movement, the sea
was three to five meters higher in relation to the
contemporary landmass of Japan than at present.
Analysis of the shells themselves has confirmed the
changes in marine topography, for it is only during
the maximum phase that subtropical species of mollusc
are present, indicating a higher water temperature.