puissance n.f. (d'une couche ou d'une série) - Epaisseur d'une couche ou d'une série de couches mesurée perpendiculairement à la stratification. C'est donc l'épaisseur réelle par opposition à l'épaisseur apparente qui peut être relevée p. ex. dans un sondage oblique à la stratification.
[Dict. de géologie, Foucault & Raoult]

Archeology is all about "stratigraphy."

Here's the term as it's used (originally) in Geology, demonstrating Bourth's technical definition:

http://www.glossary.oilfield.slb.com/Display.cfm?Term=true s...

And a recent use of it in an Archeological context:

"There is no apparent tendency for an increase in age with depth in this series of samples spanning more than 1 meter of stratigraphic thickness, ..."
http://books.google.com/books?id=1AuSnP4R-hkC&pg=PA61&lpg=PA...

Another:

"The site Varves that has already been excavated during three field seasons shows alternating levels of fine grey silt, very rich in organic and archaeological material and reaching a stratigraphic thickness of about two metres."
http://anthro.unige.ch/lap/ounjougou/finalneo.html

With illustration:

http://anthro.unige.ch/lap/ounjougou/finneo03.html

Again:

" In 1993 and 1995, two excavations were carried out at Yuchan Cave (2) in Bai Zai Chun (White Stronghold Village), in Zao Yen Zheng (Long Life Goose Town), Tao County, Hunan. Stratigraphic thickness was 1.2-1.8 m."
http://http-server.carleton.ca/~bgordon/Rice/papers/zhang6.h...

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Note added at 17 hrs (2008-03-04 13:51:59 GMT)
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"of the layer" is, of course, technically redundant, but might add some clarity, if your audience is Stratigraphically Challenged.

Another recent example:

"Chronometric Dating in Archaeology ... to Matuyama chrons occurs at a stratigraphic thickness of 900—1000 cm, about 100 cm above the Aurora stratum..."
books.google.com/books?isbn=0306457156...

While most of the Googles for the term (even when "archeology" is added to the search) are Geological, it would seem that there has been a recent trend in Archeology to expropriate it, probably because of its technical precision (as Bourth's quote demonstrates) --it's not just a matter of the *relative* layering at a site, but of the *precise* measurement (along the *vertical* face) of the stratigraphic layers. It is such precision which can allow for absolute, as well as relative, dating of the material deposited.