In the Lord of the Rings (Fellowship of the Ring), in the chapter The Fog on the Barrow-downs, the hobbits fall asleep on a saucer-like topped hill and awaken as the sun is setting. Alarmed, they attempt to march onwards, towards the northern boundary of the Barrow Downs.
This is written, as they continue their journey:
Their going was very slow. To prevent their getting separated and wandering in different directions they went in file, with Frodo leading. Sam was behind him, and after him came Pippin, and then Merry. The valley seemed to stretch on endlessly. Suddenly Frodo saw a hopeful sign. On either side ahead a darkness began to loom through the mist; and he guessed that they were at last approaching the gap in the hills, the north-gate of the Barrow-downs. If they could pass that, they would be free. 'Come on! Follow me!' he called back over his shoulder, and he hurried forward. But his hope soon changed to bewilderment and alarm. The dark patches grew darker, but they shrank; and suddenly he saw, towering ominous before him and leaning slightly towards one another like the pillars of a headless door, two huge standing stones. He could not remember having seen any sign of these in the valley, when he looked out from the hill in the morning. He had passed between them almost before he was aware: and even as he did so darkness seemed to fall round him. His pony reared and snorted, and he fell off. When he looked back he found that he was alone: the others had not followed him. 'Sam!' he called. 'Pippin! Merry! Come along! Why don't you keep up?' There was no answer. Fear took him, and he ran back past the stones shouting wildly: 'Sam! Sam! Merry! Pippin!' The pony bolted into the mist and vanished. From some way off, or so it seemed, he thought he heard a cry: 'Hoy! Frodo! Hoy!' It was away eastward, on his left as he stood under the great stones, staring and straining into the gloom. He plunged off in the direction of the call, and found himself going steeply uphill.
After they are rescued by Tom Bombadil, Frodo looks for the standing stones, but he cannot see them:
At last they set off. They led their ponies down the hill; and then mounting they trotted quickly along the valley. They looked back and saw the top of the old mound on the hill, and from it the sunlight on the gold went up like a yellow flame. Then they turned a shoulder of the Downs and it was hidden from view. Though Frodo looked about him on every side he saw no sign of the great stones standing like a gate, and before long they came to the northern gap and rode swiftly through, and the land fell away before them.
Were the two standing stones indeed the entrance to a barrow? Had the hobbits perhaps become disoriented in the fog and were moving towards the east and towards a barrow? Or, may this have been some sort of supernatural element (a mirage of sorts), having something to do with the Witch King's recent visit to the barrow-downs and summoning/re-awakening of the evil spirits?
The absence of the standing stones the next morning makes me wonder if indeed the stones were some sort of apparitional object, if not an actual barrow.