Mammoth Motocross  -
The Mammoth Motocross is held in the high Sierra town of
Mammoth Lakes CA.  Don't be surprised to see McGrath,
Dubach, Hughes or other top names in the Vet Pro and Senior
Expert class.  
Virginia City Grand Prix -
Virginia City GP doesn't have much of a web site, but you can get
the basics.  The race is a classic that starts on the main street in
Virginia City, NV.
The terrain isn't all that tough, it is the 24 hours of racing that is
the challenge.  Pull together a team or ride as an iron man.
24 Hours of Starvation Ridge  -
Pleasant Valley Raceway Winter Grand Prix -

If you like long motos, high and low speed sections and a 3 mile track, this is the
race series for you during winter in Idaho.  We need more Old Timers so take
advantage of this very enjoyable race.
Idaho Old Timers Motocross Association
A Day in the Dirt -

The video pretty much tells the story....
It isn't motocross, but if you love racing you need to run a few of the WORCS events.
Nearby races include Clay Peak in Payette, ID, Washougal, WA and Honey Lake, CA.