>>9079
> Would it be better to use 'their' instead?

It isn't 'their' magic until they absorb it.
Picture it this way - they're in a mostly constant mist, like a light rain - only it's not coming from clouds, it's coming from waves crashing against the upper, inner edge of the mountain caldera, and the shattered and diffused moisture is now slowly, irregularly landing on the pegasi's backs, which is what's powering their wings at all.
Too much flapping relative to the amount of invisible, sparsely distributed energy/rainfall, and they convert back to being earth ponies until a short time later.
Starry is absorbing magic the whole time, and mostly not using, and also is able to 'hold' more in total.

There's going to be a spot, near the ledge, where there isn't any magic, and the girls will have to be hauled up hoof over fetlock.
Then the rip-tide forces of magic-spray will try to push them right back over the edge again.