That's it. The first meaning of Russian "пласт" is layer or strata but these words can be easily confused in archaeological context.

And "так называемый" means 'so-called'.
So the text in fact says that from 1970s the focus shifted to marking out the so-called 'horizons', vast cultural and chronological uniformities of Lower and Middle Paleolithic etc.

http://www.dacc.cc.il.us/~thynes/Lecture3a.htm

Archaeological data from each site were organized in terms of their chronological age (determined through seriation studies or via the principle of superposition) and their geographical location (their spatial position in relation other artifacts, etc.; see Fig. 3.1). Comparisons across sites provided the means to develop a larger, more regional perspective, with cultural sequences divided into periods or phases, with these phases connected across sites in a region through their shared attributes or patterns to form culture areas called time-space grids; see Fig. 3.3). The concepts tradition and horizon were adopted to standardize the models produced using this approach, where tradition referred to cultural continuity through time, and horizon was cultural uniformity at a specific point in time (Fig. 3.3a).

Please, also see the article below:
http://en.wikipedia.org/wiki/Archaeological_horizon

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Note added at 2 hrs (2006-12-20 14:34:51 GMT)
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Alan, the problem is that it looks like you need one word to render both Russian 'пласт' and cultural uniformity, which I agree is quite a tricky thing. Horizon is both archaelogical and cultural and IMHO fits this criterion and I doubt that 'layer', 'stratum' or 'seam' do. Wikipedia says horizon may be used in many ways. If you put it in brackets like it is in the source text then it would be a clearly different thing from 'horizon' as 'горизонт'.



Please, see more links:

http://www.cr.nps.gov/archeology/AforI/howfig_rel5.htm

A horizon involves ties and uniformity across space at a single point in time (Ashmore and Sharer 1996:41). In archeology, a horizon is a pattern characterized by widespread distribution of a complex of cultural traits that lasts a relatively short time.

http://www.wsu.edu/~anauman/Anth 230/Lectures/Lecture 18- 19...
Horizon— primarily spatial continuity represented by cultural traits and assemblages whose nature and mode of occurrence permit the assumption of a broad and rapid spread. Horizon deals with ties and uniformity across space in a relatively restricted span of time.

Мультитран:

пласт сущ.

геофиз. geologic horizon; slab; horizon (А.Череповский, словарь по прикладной геофизике, 1998 г.)

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Note added at 2 дн22 час (2006-12-23 10:25:48 GMT)
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Just realized it, quotation marks, of course, not brackets:) Sorry for that mistake...:)

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Note added at 2 дн22 час (2006-12-23 10:41:06 GMT) Post-grading
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Thanks again:)