A glacier is a large mass of ice that lasts for years, which was formed by recrystallization of snow or hail, and that shows signs of moving, or having moved like a very slow river.
Earth goes through climate changes during which the amount of snow and ice on the planet vary.
There have been times when Earth did not have ice caps at either pole, and other times when the polar ice caps covered much of the northern and southern hemispheres and glaciers formed on all the continents. During the last ice age, which ended about 10,000 years ago, snow, ice, and glaciers covered much of the northern hemisphere. When water freezes, it fills more space than when it is liquid. Water that seeps into cracks in the ground, like the fault line on which Loch Ness is located, expands and deepens the crack when it freezes. Geologists think the glaciers that formed where Loch Ness is now are responsible for the extreme depth of the Loch.