As a result each year more snow accumulates. Snow converts to firn and then to ice.
Air is trapped in the pore spaces.
When the pile of ice becomes thick enough, even though it is a solid it begins to slowly flow. Glaciers are dynamic They need excess snow. They flow away from area of thickest accumulation They actively erode, transport and deposit rock and sediment. There have been at least seven long periods of glaciation. Each up to 10's millions of years.