Glacial cirques now ice-free. Cosmogenic dating of erratic boulders shows final deglaciation occurred 12,000 years ago, but geomorphology suggests small glaciers reformed briefly 500 years ago during the Little Ice Age.
Juan lacruz, CC BY-SA 3.0, via Wikimedia Commons
A Landscape Carved by Ice
The Circo de Gredos is a dramatic, amphitheater-shaped valley carved into the granite heart of Spain's Sierra de Gredos mountain range. This landscape, peaking at Pico Almanzor (2,592 meters), was sculpted by powerful glaciers during the Pleistocene epoch. The largest of these ice streams stretched for 8 kilometers and reached a thickness of over 350 meters, grinding out the basin that now holds the Laguna Grande de Gredos, a glacial lake at an altitude of 1,940 meters.
Today, the ice is gone, but its evidence remains everywhere. You can walk across U-shaped valleys, scramble over moraines, ridges of rock and debris pushed into place by the advancing ice, and see "erratic" boulders, which are rocks carried far from their origin and dropped as the glaciers melted. This geomorphology records the region's colder past, when a significant portion of the park was covered by ice. The main geological material of the range is granite, formed during the Variscan mountain-building event and later uplifted during the Alpine Orogeny, the same event that formed the European Alps.
Reading the Rocks
Scientists can read the history of this glaciation written on the surfaces of these granite boulders. By using a technique called cosmogenic nuclide dating, they measure the concentration of rare isotopes, like Beryllium-10 and Chlorine-36, that form when rocks are exposed to cosmic rays from space. Ice shields the rock from these rays, so the isotope concentration reveals how long a boulder has been exposed to the atmosphere since the glacier that carried it melted away.
Studies in the Gredos Gorge dated moraine boulders to between 23,000 and 21,000 years ago, marking the Last Glacial Maximum in this region. The data shows that a rapid and definitive deglaciation occurred around 17,000 to 16,000 years ago, with the glaciers likely vanishing completely from the Gredos Gorge by 15,000 years before present.
Yet, the story has a surprising final chapter. High in the cirques, close to the headwalls, are smaller, younger-looking moraines and protalus ramparts—ridges of rock that accumulate at the base of semi-permanent snowfields. This geomorphological evidence shows a brief, but significant, re-advance of small glaciers during the Little Ice Age, a period of global cooling that began around the 14th century. Lichenometry, a technique that uses the growth rate of lichens to date rock surfaces, confirms that these features formed during this recent cold snap, meaning ice returned to the Gredos cirques just a few centuries ago before disappearing to create the landscape seen today.
💡Fun Facts
The Gredos ibex (*Capra pyrenaica victoriae*) is a subspecies of Iberian ibex native specifically to the Sierra de Gredos.
Cosmogenic nuclide dating is possible because cosmic rays constantly bombard the Earth's surface, creating rare isotopes in rocks at a known rate, effectively creating a geological clock.
During its peak in the Pleistocene, the Gredos glacier was so thick—over 350 meters—that it would have buried the 158-meter-tall Torre Picasso skyscraper in Madrid more than twice over.
The Laguna Grande de Gredos, a lake formed by glacial over-excavation, freezes completely over in winter.
The park is always open, but road access to the trailhead can be restricted. The main parking area, Plataforma de Gredos, has limited space (around 150 spots), and access is controlled by a checkpoint 5km away.
Admission
Access to the park is free, but there may be a small fee for parking at the Plataforma de Gredos.
Accessibility
This is a high-mountain environment. The main trail from the Plataforma de Gredos to the Laguna Grande is a rocky, unpaved path with significant elevation change, requiring appropriate hiking gear. It is not accessible by wheelchair.
