This is primarily a bouldering area that lies in the Canyon to the East of Topanga Canyon Blvd (27). I understand there are some top rope options in the area too.
This area is described very well in Craig Fry's Southern California Bouldering Guide (2nd edition).
If you park in the town of Topanga, head south along Topanga Canyon Blvd for approximately 1/2 mile and look for a small use trail on the west (right) side of the road. This leads to a graffiti covered culvert that will take you under Hwy 27 and down to the canyon bottom. Once at the bottom, head right (downstream) for about 1/8 mile or so and you will run into what I have dubbed "Area 1". The boulders are not purple here but there is ample evidence of climbing and the problems look to be relatively moderate. Continue downstream for another 1/8 mile and you will come to the Purple Stones North area. The Purple Stones South area is found approximately 1/8 mile downstream from this area.