A varde is a stack of stones built as a trail marker or a summit register. The Norwegian varde tradition is older than recorded history - the largest mountain vardes have been continuously maintained for centuries, with each passing walker adding or repositioning a stone - and remains in active use across the Norwegian mountain country today. Above the treeline, where there is nothing for the painted T-merke to live on, the varde is the primary navigation aid. In poor visibility - fog, low cloud, blowing snow - the line of vardes becomes the trail itself.
The construction is informal but consistent. A trail varde is typically half a meter to one meter high, conical, built from local rock without mortar, and spaced close enough that the next varde is visible from the previous one in fair weather. A summit varde - placed at the highest point of a peak - is often substantially larger, with a small chamber inside that holds the topptur-bok (summit register) where walkers sign their names. The largest summit vardes - on Galdhøpiggen, Glittertind, Snøhetta - are several meters tall and maintained as named landmarks in their own right.
Cultural protocol around vardes is light but real. You do not knock down a working trail varde under any circumstances. You do not build a new varde on unmarked terrain - the unmarked country is unmarked deliberately, and amateur cairn-building is a small but real form of mountain pollution. You may add a stone to a struggling varde, particularly after a storm has knocked the upper section down. The system works because the underlying culture treats it as worth maintaining.