When a snowmobiler ascends a slope to the highest point they can reach. Similar to hill climbing.
Snowmachine High Marking Tips:
1) Start out on gentler slopes and work your way up to steeper slopes as so you can test the stability of the snow. Start on the side of a slope instead of center-punching it. Do your first runs low and fast rather than climbing as high as possible right away, which leaves you very committed and vulnerable. If possible, do your first runs on the more dangerous slopes from the top down to improve your chances of escape.
2) One at a time. If a person gets stuck, DO NOT SEND A SECOND SLEDDER TO HELP!!!! Fact: Roughly 33% of snowmobile fatalities occur when a sled is stuck. About 34% involve more than one machine on a slope at the time of the avalanche.
3) Everyone else should watch from a safe spot. Always park well away from the bottom of steep slopes or off to the side of the avalanche path. Don’t count on being able to outrun a slide, but just in case, get in the habit of parking parallel, facing away from the avalanche path, rather than one behind the other and always leave the kill switch up when you shut down your machine.
4) Be wary of steep, smooth, leeward slopes. Slopes that have been stripped by wind (windward) are usually safer than slopes that have been loaded (leeward).
5) At the top of the high mark, turn towards your escape route instead of away from it and make your turn quickly, while you still have enough speed built up to avoid getting bogged down or stuck on the turn.
6) ) If unsure of the snow stability, favor slopes that have recently avalanched over those that have not yet slid. You can still sled on unstable days — just chose slopes less than about 30 degrees that are not connected to anything steeper. On some days the snowpack is just too unstable to risk highmarking, and carving powder in the flats is the most prudent choice.
7) Avoid slopes with deadly terrain traps such as gullies, steep-sided creek bottoms, or slopes that end in depressions because of the high probability of a deep burial. Do not ride on slopes with cliffs below. Favor slopes that are fan-shaped at the bottom and do not have obstacles like rocks or trees to crash into. Concave bowls are nasty traps because the fracture propagates around the slope and all the debris collects at the bottom like a huge funnel, which could easily bury you under 10 to 30 feet of debris.