Replying to Dirty bases cleaned by grass?

Well the story goes like this. I went skiing on the last day of the season at our mountain and my bases which were originally white where tan and brown from the dirty snow. Ive been bored in the past couple days and have been doing some step on sessions (i have an edit up under rails) and just noticed that my bases were white again with an occasional grass stain which could just be rubbed off. I also tried scraping off the dirt layer before and of all things grass cleaned it. What the hell?!?