The entrance to this area begins just South of Wall SD. If your heading to the Rapid City area this will take you there instead of I-90. To drive straight through is about 1 hour. But chances are you will want to (and should) stop along the way to take in the view.
Lots of wildlife here that will pretty much ignore you as you drive past them. So there will be plenty of opportunities to take pictures.
Once inside the park you will have a choice of the paved road or a gravel road that heads off in a different direction. I've been twice and I've ended up on the gravel road both times.
Both roads do eventually meet up (consult your favorite map or online map site for more details) and you can then continue to Rapid City.
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