A geologic metronome
In the arid highlands of Granada, the Huescar endorheic basin records a remarkable history of climate change. For millions of years during the Pliocene and Pleistocene epochs, this basin was a closed system where water could flow in but not out. The result was a massive paleolake, part of the larger Guadix-Baza Basin, that appeared and vanished with the regularity of a geologic clock. The basin's sedimentary layers—hundreds of meters thick in some places—are a natural archive. These layers show a clear pattern: the lake would exist for roughly 20,000 years, then dry up, leaving behind thick deposits of evaporite minerals like gypsum and halite.
This wet-dry rhythm was not random. It corresponds to Milankovitch cycles, the periodic variations in Earth's orbit that influence how much solar radiation the planet receives. The 20,000-year pulse of the Huescar lake is tied to precession, the wobble of Earth's rotational axis. This celestial motion altered regional climate patterns, triggering wet periods that filled the basin during colder, glacial phases, and dry periods that evaporated it during warmer, interglacial times. About 500,000 years ago, the basin was finally breached by the Guadiana Menor River, which carved its way through the ancient lakebed and connected the basin to the Atlantic Ocean, ending its long history as a closed system.