Originally Posted by Bones
While I don't disagree with cleaning your base after spring riding and before storage.....
I gotta ask....where are you riding that they add "dark oily substances" to their snow base?
I thought most East Coast mountains do this. When it gets down to a really low base, you can see that the snow appears 'dirty'. It looks brownish in color, kind of an even spread over patches of slushy snow. If you don't really check it out, you will mistake it for dirty snow. Your board slows down a bit when you slide over it.
You often can't see it on your base, but it is there. Sometimes you can feel it if you slide your finger over the base. I tend to ride to the last day of the season on the last open resort, so experience may vary for people.
I've encountered the stuff at Okemo, Killington, Jay Peak and Butternut that I can remember. I'm sure many do as well, I thought it was normal for east coast snow making.