My grandfather collected the interesting rocks he overturned with his plow. He placed them in a small area under a poplar tree, and contemplated the rocks’ origins. I cannot remember what hypothesis he had for this unusual rock, but when he asked me about it, I told him that I hadn’t seen another quite like it. The rock is sandstone, about 1.5 meters in its longest direction, and has a very linear ridge about 2 centimeters high running along the length of the rock. It almost looks like someone glued a piece of decorative molding to a slab of concrete. I’m not exactly sure how it got that way, but I have a good idea.
First of all, I am fairly certain the ridge is on the lower surface of the rock. That is, the ridge side was facing downward in the sand that eventually formed the sandstone. According to my grandfather, he found the rock ridge-side-down. The ridge being on the lower surface identifies the ridge as a “sole mark”. A sole mark is any kind of structure found on the bottom side of a sedimentary bed, like the marks on the sole of your shoe. I’ve seen plenty of sole marks before, but never any this long, uniform, and straight.
I hypothesize that this is a particular kind of sole mark called a “tool mark”. As a tool (such as a stick or shell) drags along the muddy bottom of a body of water, it leaves a groove in the mud. It’s no different than a child dragging a stick along a beach, leaving behind a groove. The current moves the tool along the muddy bottom, and the cohesive mud allows the groove to persist and form a mold. If sand deposition occurs atop the mud and the groove, a sand cast of the groove forms as an inverse of the groove. After the sand and mud is cemented (a.k.a. lithified) into a rock, the shape of the groove is preserved as a tool mark. You could form the same kind of mark by dragging a stick along a muddy area of your yard, and pouring plaster of paris atop the mud to preserve the mark.
I’m not entirely sure what kind of tool made this mark, nor am I entirely certain this is a tool mark at all, but it is probably the simplest explanation for this unusual rock.