Column-style snowflakes don't usually get too large, but sometimes the conditions that favour their growth stay stable and can create larger specimens. This column has a very special additions however, in the form of additional columns growing perpendicular from the main one! How cool is that?
The "baby" columns appear to be growing in a similar way to frost. This crystal seems to differ from the examples I have for "multiple capped columns" which extrude plates from the sides, not additional columns. It does look similar to crystals photographed by Ken Libbrecht and others, but definitely carries a few questions I don't yet have answers to.
One interesting feature is the possible presence of multiple evaporation grooves. These are lines that run perpendicular to the length of the main column, barely visible along the edge. These occur when "crystal twins" are connected in a hexagonal column structure but appear rotated by a multiple of 60 degrees. They look fine, but this rotation creates weaker molecular bonds at the point of rotation which allows for some water to evaporate in this area - making a barely noticeable line or groove. I've seen these before quite commonly, but I've never seen more than one on the same crystal. Two can easily be seen, and there might even be a third. This snowflake is proving to be quite the enigma!
You might also notice the "antenna" growing out of the ends of some columns. These are pretty common on needle-type crystals where the ends will split and the corners will grow faster. The same happens with plate-type snowflakes when they grow branches; these antenna are "branches" for column crystals.
I'm sure there will be more surprises before winter is over, but this was a fun one to discover. :) Oh, and plenty of other fun mysteries explained in the Sky Crystals book: http://skycrystals.ca/