Here's what I know...
I went to see this movie somewhere between 1977 and 1980 when I was between 7 and 10. The fact it's hazy makes me think I could have been closer to 7 than 10 and I'm certain it wasn't the A-feature or I'd have remembered it. I'm pretty sure I saw it on Hamilton Odeon, which was not a cinema I went to often, which makes me think it was on either with Sinbad and the Eye of the Tiger or The Black Hole, both of which I remember seeing in there.
The movie itself had a v evocative ending and, as often happened when a B-movie was stuck to an A-movie, I might just have seen the final 30 mins. Movies like The Humanoid would be stuck before Clash of the Titans around that time or the Spidey TV pilot before Sinbad. It might not even have been a contemporary movie as they'd often just find whatever they had lying around and use it to fill time if the main movie was a little on the short side. But the production values seemed good in that Warlords of Atlantis kinda way where it was clearly shot in a studio, but still looked pretty cool.
So what do I remember? A waterfall and a group of hunters who had been looking for the Elephant's Graveyard. All the treasure in the world is in this place and they find it at the end by following this dying elephant as it staggers through the jungle, past a waterfall and into this vast place where we see the skeletons of hundreds of them here, a secret place only the elephants know about. I think the movie had a young boy as one of the leads and there even have been a monkey. But the end was a big Planet of the Apes kinda finale where you see this thing.
Does this ring any bells?"
Sounds like a Disney Wonderful World of Colour episode often shown on double bills in UK cinemas?
Could be Trader Horn?