ducking back : quickly going inside and then coming out
swathe : literally: a long strip of land of something; contextually: a land stretched with snow
careered down : descending the slope
manoeuvres : military exercises involving a large number of soldiers, ships, etc.
cairn of rocks : a pile of stones that mark a special place such as the top of a mountain or a place where somebody is buried
salt flats : thatched roofed houses covered with snow
billowed : filled with the air and swelled out