Vocalizations heard by hikers witnessed in King County, Washington

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I really don't know if this was a Bigfoot encounter or not but I don't know how else to explain it. It was in the summer around the year 1970 I was between 9 and 11 years old. There were woods all around where I lived and I spent most of my time playing in them. There was an old road passable only to bikes, horses and 4 wheel drive that led to power lines and a popular camping/party spot. It was dense forest and no direct sunlight came through the canopy. As my friend and I walked and talked all of a sudden there was a loud growling/screaming and about 50 feet or so in front of us the trees were shaking and what ever it was moved a tree (probably Alder) enough to let sunlight beam down, these were big trees, and it seemed like the earth was shaking a bit. We stopped dead in our tracks just long enough for the adrenaline to kick in and then we ran as fast as we could for the whole half mile out of the woods. It has been along time and I was young but the vision is pretty clear of the tr...