Bathtub rings on a continental scale
In the high-altitude desert of the Zanda Basin, at around 4,200 meters above sea level, the mountainsides are carved with enormous horizontal lines. These are the Zanda Basin Lake Terraces, the ancient shorelines of a massive inland lake that existed during a far wetter period in Tibet's history. The sediments that form these terraces are part of a geological unit called the Zanda Formation, which reaches a thickness of up to 800 meters. This entire sequence of silts, clays and sands was deposited on the bottom of a deep paleo-lake that filled this basin between the Pliocene and Pleistocene epochs.
The formation of the terraces records the lake's gradual disappearance. As the Himalayan orogeny continued to lift the region and the climate became progressively more arid, the lake began to shrink. Its water level didn't drop continuously but in stages, pausing for long enough periods to carve a distinct shoreline, or terrace, into the surrounding landscape. The highest of these ancient shorelines are found hundreds of meters above the current valley floor. Today, what remains of the lake sediments is being actively eroded by wind and water, forming the Zanda Earth Forest, a landscape of towering natural pillars and canyons.
A lost world of Ice Age ancestors
The fine-grained lakebed sediments of the Zanda Formation created exceptional conditions for fossil preservation. Excavations have uncovered a unique assembly of Pliocene mammal fossils, dating from roughly 5.3 to 2.6 million years ago. These fossils represent the earliest known ancestors of several famous Ice Age megafauna. Among the discoveries is the Tibetan woolly rhino (Coelodonta thibetana), an ancestor to the later rhinos that roamed Pleistocene Eurasia.
Paleontologists have also unearthed fossils of an ancestral snow leopard (Panthera blytheae) and the earliest known arctic fox (Vulpes qiuzhudingi). The presence of these cold-adapted animals suggests that by the Pliocene, the Zanda Basin had already reached a high elevation with a cold climate, being a source for species that would later thrive during the global Ice Ages. Plant and pollen fossils, including species of Cotoneaster, Spiraea, and Caragana, indicate the environment was a cool, deciduous shrubland that received more precipitation than the arid desert found there today. Isotopic analysis of herbivore tooth enamel confirms that the vegetation was dominated by C3 plants, typical of cooler and wetter environments.