[Lockbox]

It drilled through 509 metres of ice at a site called Prudhoe Dome, and pulled up 7.4 metres of frozen sediment and rock.

So drilled down 509 meters to pull up 7.4 meters to find that some melted before. I think the question is not some melted but how did over 500 meters of ice form on top?

[Dusty Road]

Last ice age the sea level dropped about 400 ft, all that water had to go some where. It got land locked as ice.