She also used a clever method called view-shed analysis, essentially jumping into the rock herself to see what it sees. What didn't matter to the length or direction of the trail was the size or shape of the rock. What did matter was its location in relationship to the terrain. The map also showed her that the trails of the rocks that moved together weren't exactly parallel, which meant that ice wasn't a factor. It dries out, but not completely.
Big winds roll through.
Hugging the terrain, the winds roller coaster in from Saline Valley and split into two tongues as they hit the hill at the southern end of the Racetrack. One tongue barrels northeast across the playa, swooping up rocks that slide in a fairly straight line. The other tongue careens around the south hill, turns as it follows the edge of the saddle, hits the Racetrack going northwest and picks up another group of rocks that also slide merrily along in a fairly straight line. Where the winds cross paths, they swirl together in a mini-tornado that picks up two or three rocks to dance Nature's version of line dancing. Low pressures created by the whirling winds might help the rocks get started.