I run across interesting pics from time to time on the net. This one indicates where it is from. I could find no email to contact the fellow and ask permission but as it has his link on it.. I think it is okay. Natural or not?
It looks manmade. As Thom pointed out "Note the apparent brick (sidewalk?) just behind it," I'm inclined to agree with him. If this is indeed part of two bricks and the grove is from an edging or grooving tool, than it has to be manmade and not natural.
Actually, the rock is natural, but one thing on it is not.
It has what looks like half a drill hole in it (see pic). It's not a treasure sign, a trail sign, or anything else meant to convey a direction. Its' just part of how they quarried the rock for roads and such.
That does kinda look like a drill hole, but I didn't suspect it to be because it looks too small. It's kinda hard to judge size in this photo, but comparing the drill hole to the surrounding snow (I know what size snow crystals are, great up in Colorado) I'm inclined to think it's NOT a drill hole. It's about 1/8th of an inch across I'd guess. Then again, like I said it's kinda hard to judge the size.
Either way, it surely is NOT some kinda marker to anything. That I am confident about.