Scree

Rocks fall from high cliffs.

They pile up at the bottom. These piles are called scree.

Water gets into small cracks in the rock. When it gets very cold, the water turns to ice. This ice grows and pushes the rock apart.

Soon, pieces of rock break off and fall. They land in a big pile. This pile can be very steep. It can even have caves inside.

Sometimes, tiny plants grow on the rocks. These plants can help break the rocks even more. This makes more scree fall down.

Scree can also cover old ice. This layer of rock can act like a blanket. It helps keep the ice cold.

Scree is a collection of broken rock fragments. These rocks pile up at the base of a cliff.

People also call these piles talus deposits.

How does scree form? It often starts with physical weathering. This is a way that rocks break apart. Water flows into small cracks in the rock. When the temperature drops, the water freezes into ice. Ice takes up more space than water. It expands by 9%. This creates a force that pushes the rock apart.

Eventually, the rock breaks and falls down the slope.

Living things can help too. Lichen are tiny organisms that grow on rocks. They can grow into small cracks. As they grow, they make the cracks wider. They also make acids that dissolve minerals in the rock.

Scree can even cover glaciers, which are large bodies of ice.

A thick layer of rocks acts like a blanket. This layer can insulate the ice. It helps keep the glacier from melting too fast. Large rocks can even form caves for people to walk through.

Scree is a collection of broken rock fragments found at the base of cliffs. These piles of stone often form steep mountain slopes.

You might also hear people call these talus deposits. The word scree comes from an Old Norse word for landslide. Talus is a French word that means a slope or embankment. Scientists sometimes use these names to mean the same thing. However, scree usually refers to the loose rocks themselves. Talus often describes the shape of the land made by those rocks.

These slopes form through a thing called weathering. This is when rocks break down over time. One common way is through freeze-thaw action. Water flows into cracks in a rock wall. When it gets very cold, that water freezes into ice. Ice expands by 9% when it turns to solid ice. This expansion creates a huge force that wedges rocks apart. Eventually, the rocks break free and fall down the slope. This process is most common during the spring and fall.

Living things can also help break rocks apart. This is called biotic weathering. Tiny organisms called lichen often grow on the rock surface. They push small parts called hyphae into tiny cracks. As the lichen grows, it makes those cracks wider. Lichen also produce organic acids. These acids can dissolve minerals in the rock. This turns solid rock into loose sediment. Over time, plants may even grow on the scree slope. Their roots can make the slope less stable.

Scree can change how glaciers behave. A glacier is a large body of ice. Sometimes, a thick layer of scree covers a glacier.

This rocky layer can act like a blanket. It provides insulation to the ice below. This can help stop the glacier from melting too quickly. However, a very thin layer of rocks can do the opposite. If the debris is less than 2 centimeters thick, it might actually help the ice absorb more heat. This is because the rocks have a lower albedo. Albedo is the ability of a material to reflect light.

Scree slopes create special tiny worlds called microclimates. These are small areas with different temperatures than the air around them. Cold air can seep through the gaps between the rocks. This keeps the bottom of the slope much colder than the top. In some places, the temperature inside can be 6.8 to 9.0 degrees Celsius colder than the outside air. These cold spots allow special plants and animals to live there. A research team led by Vlastimil Růžička studied 66 slopes in 2012. They found that these areas help protect many types of life.

Scree refers to collections of broken rock fragments found at the base of cliffs or steep rocky masses. These fragments accumulate through periodic rockfall events. When these materials form large landforms, they are often called talus deposits. The term scree can describe both the mixture of debris itself and the unstable, steep mountain slopes they create.

While the terms scree and talus are often used interchangeably, some scientists use them differently. Talus is sometimes used more narrowly for material at the base of a cliff that has clearly eroded. Scree might be used more broadly for any sheet of loose rock covering a slope. This is distinct from colluvium, which is soil or rock moved by rainwash or slow downhill creep on gentler hillsides.

The physical shape of these deposits is important. Talus deposits often have a concave upward form. Their steepness is usually related to the angle of repose. This is the specific angle where a pile of granular material becomes mechanically unstable and begins to slide. However, most scree slopes are actually less steep than this angle. They often show a concave shape where the bottom of the slope is less steep than the top. In areas with very large, boulder-sized fragments, the spaces between rocks can form human-sized passages or talus caves.

Scree formation is driven by physical, chemical, and biotic weathering. Physical weathering often involves freeze-thaw action. Water enters joints and fractures in a rock wall through precipitation or runoff. When temperatures drop below freezing, the water turns to ice. Because water expands by 9% when it freezes, it generates massive force. This force can create new cracks or wedge existing blocks into unstable positions. This process is most common during spring and fall when temperatures fluctuate around the freezing point. Chemical weathering, such as acid rain, also breaks down rocks into loose sediment. Biotic weathering occurs when living organisms alter the rock. For example, lichen grow on rock surfaces and insert tiny structures called hyphae into mineral cleavage planes. As the lichen grows, these hyphae expand and widen the cracks. Lichen also produce organic acids that dissolve minerals, further breaking down the substrate.

The evolution of a scree slope typically follows five distinct stages. These stages are accumulation, consolidation, weathering, encroaching vegetation, and slope degradation. During accumulation, loose, coarse-grained material builds up. Within the slope, sediment often sorts itself by size, with larger particles reaching the bottom more quickly. Consolidation occurs as fine-grained material fills the gaps between larger debris. The speed of this process depends on composition; clayey materials bind debris faster than sandy ones. Eventually, if weathering outpaces new sediment supply, plants may take root. These plant roots can actually diminish the cohesive forces of the slope, leading to degradation.

Scree can also interact directly with glaciers. In some cases, a thick layer of scree covers a glacier, such as the Lech dl Dragon in the Dolomites. This debris layer affects the glacier's energy balance. The thickness of the debris determines if the ice melts faster or slower. This is related to thermal conductivity, which is how efficiently heat moves through a material. A thick layer of debris has low thermal conductivity, meaning it acts as insulation.

However, the albedo, or the ability to reflect radiation, also matters. Debris has a lower albedo than ice, meaning it absorbs more solar radiation. If the debris layer is very thin—less than 2 centimeters—it can actually cause the ice to melt more quickly by absorbing heat and transferring it to the ice surface.

Beyond melting, scree creates unique microclimates. These are small environments with different conditions than the surrounding area. Because of cold air seepage, the bottom of a scree slope can be much colder than the outside air. Internal temperatures can be 6.8 to 9.0 degrees Celsius colder than the external environment. These thermal anomalies can even allow patchy permafrost to exist in warmer regions. These cold microhabitats support special plants and animals. In 2012, a research team led by Vlastimil Růžička studied 66 slopes. They found that these areas act as palaeo refugia, helping to protect regional biodiversity by supporting species far from their normal ranges.