two modern guys time travel to ancient britain--one stays

I can't remember the name of this book. It was written in "modern" times, by which I mean (me being old) they didn't have computers but they did have cars. The title had nothing to do with time travel or space and didn't sound science-fictiony or time-travelly. The time travel came as a surprise to me. Briefly, two guys on a road trip in England get diverted somehow and explore a hill or a mound or something, find a way in, and behold, they have time traveled to ancient britain. The protagonist who gets out feels compelled to go back and "rescue" his buddy, only to discover that he is now thriving as a warlord. I don't remember how it ends, who wrote it, or what the title was. I picked up a book with a vaguely "fey" presentation, got surprised by a time travel story, and came away satisfied with a critique of civilization and philosophical questions, not about the nature of time, but about the nature of what young adults need to come of age. Does that make sense? Can anyone help?

To rule it out The Paradise War?

I thought of Loose Chippings or Lost Threshold but I haven't actually read either so I can't say how closely they match your description.

>2 Aquila: Wow, that is very close! Now I want to read it! I don't think this is it though, it's too recent, and there were TWO guys who disappeared under the hill together, at first.

>3 rarm: Thank you as well; Wheeler sounds like a great author to look into and the publication date is more like what I remember from the milieu of the book. But the story I remember is definitely TWO guys on a road trip, and they aren't traumatized by a recent death in the family or anything. Just vaguely bored with getting the perfectly good lives their upbringing and education prepared them for--with all the deadening alienation that implies.

>2 Aquila: btw I am listening to a different Lawhead novel as I type: Skin Map!

>4 crzsabas: The Paradise War was published in 1991. When did you read the book you're seeking?

https://en.wikipedia.org/wiki/Song_of_Albion

>7 MissSquish: I don't remember when I read it. It was probably published in the 60s or 70s, based on the level of tech used by the characters. It was definitely not Paradise War, though. I remember the setting, plot, and mood of the book very clearly but not the author or title. :-/ Thanks for responding! I will probably rediscover it by accident in a used book store someday.

Bumping this. Who can help me remember this book?