These deposits date from the Pleistocene, i.e. the geological epoch spanning the period 2.5 million (Ma) to 11,500 years (Ka) before present (BP). Sediments infilling these valleys provide a stratigraphic link between the important sites of Dmanisi in Georgia ('DMA' in the map below) and Nor Geghi 1 ('NG1') in Armenia. These sites have already transformed the scientific understanding of the relationship between the European and African Palaeolithic.
Map: the catchments of the Hrazdan and Debed rivers, volcanoes that have erupted within the last 2.5 M a (Quaternary), currently known Palaeolithic sites and the PAGES study area.
Dmanisi and Nor Geghi 1 are unlikely to be unique so other Palaeolithic sites are probably exposed in outcrops elsewhere within the PAGES study area of the map above. These outcrops comprise stacked sediments of alluvial (laid down by rivers), aeolian (wind-blown), lacustrine (forming in lakes) and volcanic origin. The last comprises both solidified lava, e.g. basalt, and pyroclastic deposits, the result of explosive eruptions. The intimate link between archaeological deposits and datable volcanic strata offers high potential for precise and accurate dating.
Photo above: an outcrop of pyroclastic deposits at the foot of the Gutansar volcano. Previous work on the slopes of Gutansar suggests that eruptions began between 700 and 500 Ka, while obsidian produced by the volcano was a key raw material for Palaeolithic stone tool production.
The Hrazdan and Debed valleys of today are very different from their Pleistocene forebears. The gorges that characterise their central sections resulted from tectonics, the drive of the Arabian into the Eurasian tectonic plate, and have deepened with time. Vegetation likely alternated between humid woodland during warm phases of interglacials to grassland steppe in the cold glacials. Today's open environment is a result of human action during the Holocene (the last 11.5 Ka).Photo
: the present-day Hrazdan valley is a largely open landscape.