Geology for conchologists - The last 15,000 years

This article deals with the Post-glacial (or Holocene Period) and the Late-glacial phase of the last glaciation.

Subdivision of the last million years, that is roughly the time which has elapsed since the beginning of the glaciations, is effected mainly on climatic criteria. The Post-glacial Period, in which we are living, is defined as starting when world temperatures first began their steep and maintained rise at the end of the Last Glaciation, round about 8,000 B.C. In northern Europe this climatic improvement allowed birch and pine forests to invade rapidly what had previously been a tundra landscape. But the first evidences of a climatic change already appear rather earlier than this, about 12,000 B.C.: an episode with a transitional (late-glacial) climate can be recognised, given a very distinctive character by the occurrence within it of a well-marked but temporary climatic improvement known as the Allerod Oscillation. The Late-glacial Period thus comprises two rather cold phases (Older and Younger Dryas), separated by a cool-temperate interval (Allerod), which can be regarded as a kind of false dawn to the Post-glacial Period.

The course of climatic change during the Late-glacial and Post-glacial Periods is known in considerable detail from studies of fossil vegetation preserved in the deposits. Investigations were at first confined largely to leaves, seeds and fruits| in more recent years a close study has been made of the fossil pollen grains abundantly preserved in peaty deposits. The grains are identified, their relative frequency at different levels counted, and hence a clear picture obtained of changing forest compositions. Pollen zones numbered from I to VIII have in this way been established for most of Europe. The changes of vegetation are of course not identical over wide areas, due to differences in latitude, but in spite of this a striking parallelism has been proved in many parts of the world. That the climatic changes which determine the boundaries between most of the pollen zones are synchronous has been shown by the technique of radiocarbon dating, whereby an absolute age can be assigned to samples of suitable organic material up to a maximum age of about 50,000 years.

Erom about 3,000 B.C. onwards in Europe the dating and correlation of Post-glacial deposits becomes particularly difficult. Climatic changes have been relatively slight; on the other hand human interference for agriculture becomes increasingly important, so that vegetational changes tend to be of local significance rather than of regional.

The Post-glacial history of the land and freshwater Mollusca in Britain appears to be broadly similar to that established for the flowering plants; in other words, as temperatures rise, artic- and boreal-alpine species disappear, and warmth-loving species move in. By the beginning of the Atlantic Period (zone Vila), when Britain was cut off from the continent¬al mainland, most of our species were already present. The most important changes since that time have been those produced by man. Woodland and marsh species have become increasingly rare and local, whereas many xerophiles and open country forms have prospered and spread. Also a number of species have been accidentally or deliberately introduced from abroad, to become familiar members of our fauna? notable among these is probably the common garden snail, Helix aspersa, unknown in this country from deposits of pre-historic age.

The appended diagram includes terms in common use for the subdivision of the Late-glacial and Post-glacial Periods, and shows the broad climatic and vegetational changes.