Permanently frozen rivers of ice found in some mountains are known as:

Correct Option

The correct option is

Glaciers

Explanation

Geomorphic agents are mobile mediums capable of removing, transporting, and depositing earth materials. In high-altitude and high-latitude regions, large masses of moving ice act as significant geomorphic agents.

Option Analysis

• Horsts: Incorrect. A horst is a raised fault block bounded by normal faults. It is a tectonic landform (block mountain) resulting from crustal movements, not a formation of ice.
• Glaciers: Correct. Glaciers are large, perennial masses of ice that move slowly over land under the influence of gravity. Due to their flow and channelized movement in valleys, they are frequently described as "rivers of ice."
• Graben: Incorrect. A graben is a depressed block of the Earth's crust bordered by parallel faults. It typically forms a rift valley, which is a structural landform rather than a glacial feature.
• Terraces: Incorrect. Terraces are bench-like or step-like landforms. They can be formed by rivers (paired terraces) or marine action, representing former levels of valley floors or shorelines.

Key Takeaway: Glaciers are moving bodies of ice formed by the accumulation, compaction, and recrystallization of snow, serving as primary agents of erosion and deposition in cold environments.