A pit crater (also called a subsidence crater or collapse crater) is a depression formed by a sinking or collapse of the surface lying above a void or empty chamber, rather than by the eruption of a volcano or lava vent. Pit crater chains are distinguished from catenae or crater chains by their origin. Pit craters usually lack an elevated rim as well as the ejecta deposits and lava flows that are associated with impact craters. Pit craters are characterized by vertical walls that are often full of fissures and vents. A tarn (or corrie loch) is a mountain lake or pool, formed in a cirque excavated by a glacier. A moraine may form a natural dam below a tarn. Alpine high altitude alp climate highland sediment sediments crater mountainous hilly peaks sharp rocky rock wall summits snow ice moraine cliff fly flying glaciation. Dangerous summit and cliffs. 4K UHD 3840×2160 30p.