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.