A Library of Ice Ages
In the fertile Fergana Valley of eastern Uzbekistan, road cuts and natural bluffs expose a remarkable terrestrial archive of past climates. These are the loess-paleosol couplets, thick sequences of wind-deposited silt (loess) alternating with ancient soil layers (paleosols). The loess deposits in this part of Central Asia can reach thicknesses of up to 40 meters. Some of the most extensive loess sequences in the wider Central Asian region, such as in neighboring Tajikistan, are up to 200 meters thick and began accumulating as early as 2.5 million years ago.
Each couplet in the sequence represents a complete glacial-interglacial cycle. The thick, yellowish loess layers consist of fine silt blown from deserts and glacial outwash plains during the cold and windy conditions of an ice age. When the global climate warmed and conditions became wetter during an interglacial period, vegetation grew on the stable loess surface, forming a dark, organic-rich soil—a paleosol. As the climate cycled back to a glacial period, a fresh layer of loess buried the soil, preserving it for millennia. This process, repeated over and over, created a striped pattern in the earth, a record of planetary climate shifts. The Fergana Valley sequences provide a detailed record of these changes, particularly since the beginning of the Brunhes Chron, the geologic period of the last 780,000 years defined by the Earth's current magnetic polarity.