Matt and SDCFIA,
There are absolutely man made monuments.
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Those are two monuments in one particular canyon that led to a sealed mineshaft in SoCal. I know it was Spanish because of a rotten leather sack of gold and silver coins I found inside.
Mike
Of course there are man-made monuments - they are primarily rock cairns, blazed trees, rock carvings, etc. and are obviously not naturally occurring. They were created for a dozen different reasons by explorers, trappers, hunters, pioneers, ranchers, hikers, miners, surveyors, sheepherders, soldiers, hippies, homesteaders, pranksters and who knows who else. They were created to be seen, ie easily found - not subtle. A very small percentage of the monuments you may find - very small - may even be associated with something hidden. If so, their meanings will not be obvious.
Your two monuments are obviously man-made - stacked rocks and a carved arrow. That's why you found them. Whether or not they led to Spanish gold ... well, that's your story, but for the purpose of the question, irrelevant here. What is relevant is that a man-made monument will show obvious evidence of being created by people for a specific purpose.
Now, carved objects. As Matt says, if you find something you think was shaped by humans to resemble something else, you'd better also find evidence of human work on it, otherwise it's just an interesting rock. There are thousands if not millions of them out there - especially faces (pareidolia). Below are two examples.
Here is a rock that looks like Richard Nixon from this angle. It sits directly above a site that is well-known for its "treasure carvings". (Coincidentally, the site also registers perfectly with other landmarks on one of the "Peralta Stone Maps".) Did somebody carve this thing to alert others? No, it's totally natural - no evidence of tool marks, non-patina surfaces, etc. Could someone use the facial profile as a landmark? Sure, if you find the proper angle it might be something to look for.
Below is another "treasure carving" site associated with the first one above, but several miles away. The balanced rock looks remarkably like a long-eared dog from the trail in the bottom of the canyon. The dog is looking directly at some hard-to-get-to petroglyphs on the top surface of the flat-topped rock to his right - impossible to see without climbing up to them. Did somebody carve this dog to alert others? In this case, it's quite likely. The fractured rock surface to the right of the dog's ear appears to have been chiseled or spalled away, leaving a fresher darkened surface exposed, forming the ear and eye socket. Then his eye was easily formed, again with evidence that part of the original rock was chiseled or spalled away. The rest of the original rock seems untouched. The last picture shows the petroglyphs.
