Bedrock polished smooth by moving ice. Scratch orientation analysis shows ice flowed from three different directions over time, recording shifting glacier flow patterns during the last glaciation.
Moahim, CC BY-SA 4.0, via Wikimedia Commons
A Record Scratched in Stone
In the Ordesa y Monte Perdido National Park, a UNESCO World Heritage site, the bedrock tells a story of immense pressure and slow, grinding movement. Here, extensive surfaces of limestone and dolomite, originally formed during the Cretaceous and Eocene periods, were smoothed and polished by the colossal weight of ancient glaciers. This geological feature, known as glacial polish, is common in landscapes shaped by ice. What makes the polish here exceptional are the fine, parallel grooves etched into the surface. These scratches, or striations, show, preserving the exact direction of the ice's flow.
The process begins when a glacier, a river of ice, flows across the land. It picks up and embeds rocks and sediment at its base, turning its underside into a gigantic sheet of sandpaper. As the ice moves, this abrasive layer grinds against the bedrock, pulverizing it into fine powder and leaving behind a smooth, polished surface. Larger, sharper rocks embedded in the ice cut deeper, creating the distinctive striations that geologists can read like a frozen map. These features are evidence of the size and movement of ice sheets that have long since vanished.
Decoding an Ancient Ice Cap
The glacial polish in Ordesa has a complex history. Analysis of the striations reveals not one, but three distinct sets of scratches with different orientations. This shows that the ice flowed in three different directions at various times, a record of the shifting dynamics of the Pyrenean ice cap during the last glacial period. This period saw its maximum extent in the Pyrenees not during the global Last Glacial Maximum (around 26,000 years ago), but much earlier, during a phase around 74,000 years ago.
These shifting flow patterns document the evolution of the ice sheet. An initial flow might represent a smaller, localized mountain glacier. A subsequent set of scratches in a different direction could record the arrival and movement of a much larger, regional ice sheet that subsumed the local glaciers. A final set of striations may then indicate a change in flow as the ice thinned and its movement was once again controlled by the underlying topography during its final retreat.
The glaciers that left these marks were immense. At their peak, they were up to 480 meters thick in the main valleys and carved the landscape on a massive scale. The classic U-shaped profile of the Ordesa Valley is evidence of the erosive power of these ancient ice flows. The polished and striated bedrock provides a micro-scale record of these same enormous geological forces.
💡Fun Facts
The name Monte Perdido means "Lost Mountain" because its 3,355-meter peak is not visible from the French side of the Pyrenees.
The present-day Monte Perdido Glacier is a remnant of the massive ice sheets of the past, but it has lost over 92% of its area since the end of the Little Ice Age.
The oldest ice dated on Monte Perdido is 2,000 years old, meaning it survived both the Roman and Medieval Warm Periods.
The park hosts over 171 species of birds, including the lammergeier, or bearded vulture, which can have a wingspan of nearly 3 meters.
The park is open year-round, but access to certain areas is seasonal. In summer, Easter, and on some public holidays, access to the Ordesa Valley floor is by a mandatory bus from the town of Torla-Ordesa to control traffic.
Admission
Free. The bus service requires a paid ticket.
Accessibility
Seeing glacial polish requires hiking on mountain trails, which can be steep and uneven. The main valley floor trail, leading to the "Cola de Caballo" waterfall, is a long but relatively flat and accessible walk.
