It was what I believe to be a hoar frost. Infomory.com gives this explanation of hoar frost: "The deposit of ice crystals that form on vegetation or other objects that are commonly mistaken to look like snow, hoar frost has many different names, each of which depends on where the hoar frost is formed. Air hoar is those that form on object surfaces. Examples of these are tree branches and stems of plants. Surface hoar looks like ice crystal ferns that are found on already frozen surfaces. They are caused by avalanches. Crevasse hoar are those that are located in glacier-like crevasses wherein water vapor forms even when there are calm weather conditions. Depth hoar is made up of cup-shaped crystals formed inside dry snow.
This kind of frost also forms around artificial environments such as industrial cold-storage facilities. Rooms that are not insulated against the cold or other areas where humidity and moisture enters and freezes instantly also form hoar frost.
The term hoar frost is believed to have come from a German word that means graying and old. The term expresses the white color of frost that is similar to the color of an old man’s beard.
The popularity of hoar frost increased due to its distinct appeal. Ragged and yet mysterious, hoar frost reminds people who take a closer look of a lacy ice crystal. The minute detail and intricacy of the design of the hoar frost makes it a very popular subject for contemporary artists. Hoar frost, when seen, most of the time gives off the feeling that one is in a winter wonderland."