thumb|250x250px|Large dirt cones on Sólheimajökull, Island
A dirt cone is a type of depositional glacial feature. Dirt cones are not actually made entirely of dirt. They have a core of ice, snow, or firn that gets covered with material and insulated. The material, if it is thick enough, will protect the underlying core from ablation. The thickness of material needed to insulate the core is called the “critical thickness.” If the material is less thick than the critical thickness, it will actually speed up erosion of the core through ablation. This is called “indirect ablation.” The cone would then begin melting and shrinking away.
Formation
none|thumb|347x347px|Formation of a dirt cone on a glacier
Dirt cones begin forming in a crevasse or a hollow. Dirt, dust, or moraine deposit material will fall into the crevasse in the glacier and build up over time. At the same time, the surrounding glacier lowers through ablation until the dirt filled crevasse is exposed and the material begins to spread out of top of the glacier. The rest of the glacier continues to lower as the material mound grows higher and taller. Any ice, snow, or firn trapped under the material will be insulated and protected from erosion. It begins forming a conical shape as the sides steepen to an angle that is unstable. Material falls down and protects the sides. The more material is added to the top, the more insulated the core becomes. Over time, it becomes a cone with a layer of material on the outside and a core of ice, snow, or firn on the inside. The material at the top of the cone is generally thicker than the material on the sides of the dirt cone.
Some cones are only a few centimeters tall, while others can be up to 30 meters high. The larger dirt cones are commonly multiple cones that have fused (melted) together. As an ice sheet or glacier melts and retreats, the cones on the surface are brought together and eventually fuse together. This strengthens the ice core and creates a thicker layer of material on the outside to protect the hardened core from eroding. They can develop in one winter season or they can be a product of several seasons. All they need to form is debris of some sort and ice.