Kell Morris does not remember exactly how he started tumbling or how he ended up on his stomach, but he remembers when a 700-pound boulder hit his back — the jolt of pain it caused as it pinned him down, and how instantly he knew he was in trouble.

Mr. Morris, 61, said it was a “beautiful, beautiful day” on May 24 in Seward, Alaska, where he lives with his wife, Joanna Roop.

Wanting to avoid the Memorial Day weekend crowds that clog up popular trails, the pair decided to hike near a remote glacier more than 120 miles south of Anchorage. It was a route they had taken before, and it was one they knew would be empty.

They had been looking for a spot to cross a creek when the earth holding a group of boulders gave way and began sliding down. Mr. Morris, who had been standing there, started sliding, too. He said he tried to almost surf the sliding gravel but lost his footing and tumbled down.

“The next thing I know, I’m face down in the creek and you can still hear these rocks,” he said, describing the clattering of falling rocks as “kind of a scraping and hitting at the same time, but it’s much, much deeper.”

Ms. Roop, 61, who was several yards away looking for a good spot to cross the creek, also heard that distinctive sound.