An Ice Age at Madrid's Doorstep
The landscape of the Lozoya Valley, just north of Madrid, holds clear evidence of a dramatically colder past. During the Last Glacial Maximum, approximately 21,000 to 27,000 years ago, the peaks of the Sierra de Guadarrama mountains hosted a network of glaciers. The most significant of these originated on Peñalara, the range's highest peak at 2,428 meters. This main glacier flowed down into the valley, creating a river of ice that sculpted the terrain.
As the glacier advanced, it scraped and plucked enormous rocks from the mountainside and carried them downstream. When the ice eventually melted, these rocks were left stranded far from their origin. Known as glacial erratics, these boulders are geologically distinct from the bedrock where they now rest. The erratics in the Lozoya Valley are primarily granite and gneiss, matching the rock types found higher up in the Peñalara massif. The glacier transported some of these boulders more than 15 kilometers from their source.
Reading the Glacier's Past
Geologists reconstruct the size and power of ancient glaciers by studying the debris they left behind. The size of an erratic boulder is directly related to the thickness and velocity of the ice that carried it. Only a truly massive glacier could move the largest rocks found in the valley. Analysis of the size and distribution of these granite and gneiss erratics shows that the ice here reached a thickness of at least 200 meters, and in some parts of the Peñalara cirque, it may have been up to 300 meters thick.
The end of the glacier is marked by moraines, long ridges of rock and sediment deposited at the ice's edge. By dating the boulders on these moraines, scientists can create a timeline of the glacier's advance and retreat. Using cosmogenic nuclide dating, which measures how long a rock has been exposed to cosmic rays, researchers have determined that the main glacial advance occurred between 31,000 and 19,000 years ago. A rapid and final deglaciation of the area happened around 17,000 to 16,000 years ago, signaling the end of the ice age in central Spain.