An ice age island
Rising to 2,314 meters (7,592 feet), Moncayo's San Miguel peak is the highest point in the 500-kilometer-long Iberian System. Its altitude creates a climatic island, with cold winters and seasonal snow cover lasting from October to May. During the last phases of the Pleistocene Ice Age, this peak was not buried by glaciers like those in the Pyrenees, but it was a zone of intense periglacial (or "near-glacial") activity. This activity was driven by constant freeze-thaw cycles that shattered rock and moved debris on a massive scale.
The most intense period of this activity corresponds with the Younger Dryas, a cold snap that occurred between roughly 12,900 and 11,700 years ago. This event disrupted the post-ice-age warming trend and plunged regions of the Northern Hemisphere back into near-glacial conditions. On Moncayo, this meant the ground was permanently frozen at depth (permafrost), with only a shallow surface layer thawing each summer. This environment created a suite of landforms that remain etched on the mountain today. The mountain's geology, dominated by Paleozoic slates and quartzites, provided the raw material for this icy construction.
A landscape shaped by frost
The most prominent relict landforms on Moncayo are its glacial cirques, armchair-shaped hollows carved by ice. The San Miguel cirque is an example of this past glacial presence. Below the cirques, the landscape is strewn with evidence of periglacial processes. Block streams, which are vast fields of angular boulders, choke the valley floors. These formed as intense frost-shattering broke the bedrock apart, and the debris slowly moved downslope over the frozen substrate.
Fossil rock glaciers also mark the slopes. These are not pure ice glaciers, but rather a frozen mixture of rock debris and interstitial ice that once crept downslope at rates of a few millimeters to meters per year. Though the ice that once mobilized them has long since melted, their distinctive lobed shapes and ridge-and-furrow surfaces are still visible. Other, more subtle features include patterned ground and solifluction lobes—sheets of soil and rock that slid gently downslope when the active layer thawed in summer. Together, these features make Moncayo a vast, open-air museum of the climatic turmoil that marked the end of the last Ice Age.