Original post where the mystery is offered. If you’ve not read it already, check it out and then come back here.
Given the uniformity of the trail, my best guess at this time is a gastropod of some sort. Perhaps the periwinkles or limpets that are common at this upper level of the beach. I can’t figure out how they would leave a trail of sand, however. It was not a windy day, so there’s no way I can think of the wind would have blown on a moist trail. The other alternative that comes to mind requires a fair supply of sand sticking to our limpet or snail and being deposited in a fairly uniform fashion as it moved along its several inch path. It’s not clear to me how (or why) the creature would be packing such a supply.
The timing of the track is also a bit of a mystery, though solving it might provide a clue to the ‘how’ question. My first thought was the sand must have been deposited after the water dropped below this section of beach. It seemed obvious enough that a sand trail like this would not survive immersion. However, if I question this assumption, perhaps there is a way for sand to adhere despite being submerged. If the creature laying the trail produced an actual slime trail that held together for a time even when underwater, the sand lifted into the water column by gentle wave action (it was a calm day) could stick to the slime trail. Once exposed to air by the falling tide, the slime could dry and leave a dry sand trail with no obvious means of creation. This possibility actually seems more likely to me, though I still don’t know what created the trails. Some measurements of the gastropods on that part of the beach could narrow down my options a bit.