Interesting drip pattern on my windshield

When driving into work today, I noticed an interesting way a raindrop ran down my windshield. It's a little difficult to see in this picture, but it would drip down in little steps; sometimes going more across than down, sometimes more down than across. I'm talking about the left-hand fork of the drip.

The angle is a little wonky, but you can sort of see it clearer here (it's the drip on the right fork here):

The "across" paths basically track the path of my windshield wipers. I wonder if sand or salt or something got dragged across the windshield by the wipers, and basically etched micro grooves into the windshield that channel the water.