Carbon Canyon Regional Park is located in Brea, Ca and was opened in 1975 after it was determined that the area should be preserved primarily for flood control purposes. I'm not able to confirm it, but the name probably came from the abundance of oil found in the area years ago. You can even still see a few remaining oil rigs dotting the hill sides. The mountain biking isn't in Carbon Canyon but in Chino Hills State Park that is adjacent to it, but for easy parking and access just enter through Carbon Canyon and pay the $2.00 toll M-F and a bit more on the weekend. To get to the trailhead , you head east along the Carbon Canyon Rd. until you see the signs that take you around a small gate and to the trailhead below.