What you captured is rime but it also goes by "frozen fog deposits", "frost feathers", "ice feathers" or "feathers of rime". All are correct but just stating they are rime is easier and correct as well. They form on the windward side of mountains that were in fog that was below freezing. The feathers grow into the wind and are a great way to see how wind is being deflected off various surfaces. The feathers you saw are just like the ones we get on the summit. Sometimes these feathers can be several feet long. They form when supercooled water droplets the size of nanometers that make up the fog hit another surface that is below freezing and freeze on contact. The difference between rime vs glaze is mostly color as both can create feathers although it is less common to see with glaze. The white color is from oxygen being trapped in pockets while the rime is forming. Because they freeze so rapidly, there is very little holding the feathers together which is why they break so easily when forming. It isn't until they start to partially melt that they become hardened.
How do they form? It is easier to draw then explain but I will try. When water freezes it does so in a linear fashion. Snowflakes typically have six sides based on how water molecules bond therfore the needles stick out further than the sides of the hexagon because they formed on the points of the hexagon typically in straight lines away from the center. Rime starts on a surface as a single point that starts to grow out as more molecules slam into that point. Then, through various alterations of airpressure, air flow, temperature, humidty and imperfections forming on the riming surface, other molecules start to cohere to the "sides" of the rime that has already stuck to the surface. So, if it were forming in a x, y, z plain where y is a pole lets say and x is the gound below it with z transecting the plain. The rime starts to grow more or less parallel with x as it grows out but the feathers begin to grow out the sides in the z direction as they bond with the rime forming out. This creates a "t" shape for the rime but then rime starts to form on the extending arms in the z plain creating several more x, y, z plains that rime are forming on of these plains. So the rime starts to take on a 3-d growth pattern. There comes a limit to how far out these structures can form based on location, wind speeds, temperature, and cohesion when forming. Because the bonds are relatively weak between the various branches, if another bit of rime shears them, they break. If winds shift, they break. If they start growing into each other limiting the amount of droplets slamming into them, they stop growing in that direction or slow their growth in that direction (hence why sometime one side is longer than the other). Hopefully this all makes sense, it is easier to draw then explain as a picture is worth 1000 words or more.
Staff Meteorologist/Night Observer, KMWN (Mt Washington Obs., NH)