Although I've seen a glacier first hand for about 30 minutes of my life (the JungFrau in Switzerland), I think the following may tidy up some of the Q's going on here...
The primary force behind glaciers moving downhill is gravity.
Increased pressure on ice decreases its melting point, thus liquefying it at the pressure point IF there is sufficient pressure - the skating reference is very apt. It may well be that an ice sheet has to be 100' thick for this to happen - I don't know.
Consequently, a glacier's downhill velocity will not only be a function of the slope it's on, but also the thickness of the ice (and hence the volume of water/lubricant at its base. It's conceivable that the downhill velocity may change from season to season, but predominantly (and this is a wild guess) as a function of the amount of ice being deposited upon the head of the glacier - its nevee field. More ice on the nevee field, bigger push from the top, therefore greater velocity in winter????????? A counter argument would be that the toe of the glacier melts more in summer, therefore causing an imbalance between head and toe, and therefore a greater velocity in summer. Either way, there WOULD be a degree of backwards and forwards movement with the toe between the seasons - couple with increased meltwater at the toe druing summer. This will add to the erosion of the boulders in the morraine, but bear in mind they have already travelled many kilometres during which they were being constantly ground.
Ice itself is not hard enough to grind the rocks within the glacier or the mountains and valleys surrounding it. The carving is caused by rock acting upon rock - at all scales. The ice or meltwater carry the agents of abrasion into contact with each other and carry away the products. Nevertheless, sundry processes such as frost wedging would apply.
Another thing to think about is the sheer weight of great ice fields such as those in Antarctica and Greenlan. In both cases the ice is several kilometres thick and has thus downwarped the Earth's crust underlying it. As the ice melts (as one day it will - though we wont see it), the rock will rise up again - like when you poke a sponge cake then take your finger away.
I've gotta say I'm sceptical that the aircraft "sank" 250' into the ice. I'm sure it sank some, but I reckon 240' of snow and ice have fallen on it since it landed there 50 years ago. Happy to be wrong on this though!!
stales
