Possible howls heard 4 miles north of Rochford in middle of Black Hills witnessed in Pennington County, South Dakota

In the summer of 1973, I had driven up to the Cement Ridge Fire Lookout, since I had never been up there, and was curious about the place. I was parked in a nearby pull off area, and just walking around, enjoying the nice day and the scenery, when I heard a huge noise, something like a cross between a bellow and a roar. It did not sound at all like the scream of a mountain lion, since it was too loud and deep for that, and not in the appropriate range of vocal timbre. I was thinking that maybe a cow had broken its leg and was bellowing in pain, but it also didn't seem like a cow's vocalizing. It went on, repeatedly, for some minutes, and then I got an eerie feeling, and having no weapon with me, I decided I needed to get out of there. As I was driving back down the road, toward Rocheford, I heard the noise several times again, as I went down the road, even after a couple of miles. Many years later, I happened to be watching a Discovery Channel program on television, featuring a Native American man from the Puget Sound area of Washington state, who claimed to have made recordings of Sasquatches. He played one, and I just literally froze, because it was exactly the same sound that I had heard up on Cement Ridge, in the Black Hills, in the summer of 1973. The tone, the same loud bellowing/roaring quality, the sound was unmistakable, since I had only ever heard it once before. I to this day believe that I heard a Bigfoot screaming, because the researcher's recording reproduced that sound, precisely, and recalled my memory of it, immediately, beyond question.