Pillar Rock campsite is one of the most important sites along the entire Lewis and Clark trail, it being where, on November 7 1805, the party believed they had finally reached the ocean (they actually had another 20 miles to go). Despite the promising signs pointing the way to this historic site, it is marked only by two boards on a nondescript turn out, one of which reads: "This is the closest, most accessible location to the campsite. The campsite area is located on private property and it is not open to the public under any circumstances."
If you go:
Turn off Washington SR4 at Rosburg and follow the Altoona-Pillar Rock Road for about 5 miles. Prepare to be disappointed!
Links:

Altoona-Pillar Rock Road
Rosburg, Washington 98643 USA