Natural Science Forum / Earth Science / Oceanography / July 2005

Why do you think that?

There is also lake varves

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http://www.mnsu.edu/emuseum/archaeology/dating/dat_varve.html

Varve analysis is the process of counting varves or annually laminated
sediments to determine the rates of change in climate and various
ecosystems. Varves form when glacial advances come in contact with
bodies of water such as lakes. When this process occurs, layers of
sediment form on the floor of the body of water. This technique
provides an opportunity to acquire detailed chronological information
about the composition, displacement, and climate of that region, at
that time. It was first developed by the Swedish scientist Baron de
Geer in 1878.

Varves form distinctive layers. A varve consists of two layers; a thick
light colored layer of silt and fine sand which forms in the spring and
summer and a thin dark colored layer of clay forming in the fall and
winter. Since low temperatures are necessary in delaying the settling
of clay particles, it is assumed that varve formation can occur only in
glacial waters, particularly lakes on the margins of glaciers. The salt
and electrolytes in seawater cause a homogenous mass, preventing the
formation of varves.

Traditionally, when one "counted" varves, it proved to be a fairly
monotonous and tiresome task resulting in much time lost and much
subjectivity due to determining layers. One had to take microscopic
evaluations of the material and form some sort of hypothesis or
interpretation from the shape and thickness of the layers. This
provided inaccurate data because different varve types can cause
mistakes in the algorithm that is used and produce miscounts. For
example, sub-annual laminations can be counted as extra years, and
certain years may not deposit a carbonate layer, which often would not
be counted. Thus, this technique is much criticized for providing
insufficient data to determine such changes.

Significant advances have been made to improve accuracy in determining
varves. Using audio/visual equipped computers and digital video
cameras, differences in hues can be determined and analyzed. Through
such a procedure, accurate counts are made to determine different
layers, which in turn gives varve analysis more credibility. Also, with
the aid of computers and digitally enhanced photography, content and
classification are accurate and efficient.

Here is something else you might want to explore

Relative Dating
http://www.gpc.edu/~pgore/geology/historical_lab/relativedating.htm

Most people whom have a problem with all the various dating methods
tend to have a particular conclusion in mind, which requires they
exclude any evidence which does not support it.