Re: Fossil water
"Pressure solution"
Ice this thick weighs enough to depress the continental crust below it. However the interface between bedrock and glacier is not planer but very irregular. High pressure points are created in prexisting, structurally controlled, bedrock depressions. These pressure points are great enough to melt the ice to water.
In turn, the water lifts the ice hydrostatically, vastly reducing the friction of the ice-bedrock interface, allowing the ice to move down very gentle slopes.