Hidden Evil Desert Dimension and Ankh Necklace in 70's Film

by Russ
(Boston, Massachusetts, USA)

I've been searching for this 70's B-movie I saw as a child. It's stumped everyone I've asked.

Some teens/twenty-somethings (possibly in the UK) had stumbled into a hidden evil alternate dimension in the desert. On one side, of the barrier everything was normal. On the other side, everything had a reddish or dark tint. People on the normal side couldn't see what was happening on the red/evil side.

At the end of the movie, one of the unfortunate youths (a male) dies on some rocks on the evil side. He's holding a small silver ankh dangling from a thin black rope necklace in his hand. I think one of his friends was on the normal side, maybe 20 feet away, but couldn't see him.

Any ideas?