Replying to Mountain Biking Portland?

I've heard rumors that there are a few ok trails running down through parks in portland. One near lewis and clark, a down through washington park. Anybody got any more info on trails in and around portland? I just got a specialized SX trail and would love to do some rides without driving an hour... Also what do people think of the spots around hood and blackrock park? I have done a lot of riding in colorado and southern utah but nothing out here yet. help a brother out