Take the left down, and then follow the trail according to the trail map. You end up going down to the left, and make sure you stay on the main road. Make sure you speak to the folks who run Mt. Waterman, and we got lost the first time down it. It's a fast and sandy fireroad, and the hardest part is biking back up to the liift on the road bacause you're at altitude.
Take California Highway 2. Go 38 miles north of La Canada Flintridge. Here you will find a ski lift called Mt. Waterman to your right. Park where you can. Take the lift up. It's either $15 for the whole day, or $5 each time on the lift.
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