What is the opposite of "impression"?

With the arrival of long days and spring heat, the ice on the lakes and ponds won't be around for much longer.

I'm in Beaver Creek now. On my way here, I noticed some tracks running across a small lake.  That's not unusual, but these tracks were.  You see, the walker compressed the snow in such a way that the surrounding snow melted faster than the footprints, leaving an elevated trackway.

Very cool.