I loved Fantasia as a child, and still do - but I have to admit that this sequence was pretty darn scary. (Disney did include it years later in their Halloween Treat TV special, after all - remember that? I used to watch it every year.) Anyway, a quick summary of the animation: In the dead of night, the huge, craggy mountain looming over a small European town becomes the haunt of a huge winged demon and the gathering place of ghosts, ghouls, devils, witches, and all manner of fell creatures. They proceed to have a fiery revel, but are forced back to their lairs by the tolling of church bells.
While the animation is truly imaginative, it's the music that brings me to a geological reference - and the setting that inspired Mussorgsky's title. (Or, rather, Rimsky Korsakov's orchestral arrangement of Mussorgsky's original tone poem St. John's Night on the Bare Mountain.) A bald or bare mountain, in East Slavic (and especially Ukranian) mythology, is a place where witches and other evil creatures gather for their Sabbath. In Ukranian, the term is lysa hora, which is also the name given to many "bald" mountains in Ukraine and Poland. One of the most famous is the Lysa Hora in Kiev, which was supposedly the site of some of the largest witches' gatherings.
This got me to wondering: Why would these hills be bare in the first place? Perhaps there's some geology at work here. In the Blue Ridge mountains, for example, there are a number of spots where the mountaintops are rocky and relatively bare of trees; Old Rag Mountain is topped with granite, and Blackrock Summit with well-cemented quartz sandstone. I was able to find this map of the geology of the USSR (from 1944), but I can't read or type Cyrillic, so it's not much help. Ole Nielsen has a post on kimberlites in the Ukranian Shield which mentions granites, an excellent candidate rock type for bare-topped hills, and possibly what one might find on the summit of a lysa hora. Why would these rocks form bare summits? Well, granite can take an extremely long time to form enough soil to support plant life, especially if it's exposed to transport processes by being at the top of a mountain - and while a sandstone might weather more easily, it needs some organic inputs before it will form anything but...well, sand.
So, it's a good bet that the impressive winged demon in the Fantasia sequence is clinging to something granitic, although the craggy peak in the film is showing just a teensy bit of vertical exaggeration, if you ask me.