I grew up in a mountain valley in southwestern New Mexico near the Mexican border. All of the hills and even the mountains there were covered with rounded rocks, river-run type of rocks. I noticed the same thing in Southern Arizona. So how were these rocks rounded? Was it glacial action in some far distant ice age or what?
I gotta go with, “They were rounded in a river that isn’t there anymore.”
Do glaciers round rock? I would think that a glacier would more likely grind a rock flat on at least one side rather than round it. But I have to admit this isn’t a subject that been on my mind a lot.
They used to be pointy, but time makes fools of us all.
sand is abrasive
Some rocks get deposited in odd locations by plate tectonics. Millions of years ago they would’ve been in a river, but through mountainous shifts in the earth’s crust get pushed into new locations. I saw a documentary that mentioned that recently, so I’m not just making it up.
There weren’t glaciers that far south during the last glacial period, but the entire west was a lot cooler and wetter. Many valleys that are desert with only intermittent streams during the current inter-glacial period might have been semi-arid with permanent rivers during the glacial periods.
nature abhors a corner.
Aside from transportation that tends to round sharp corners of a rock, there is also spheroidal weathering wherein exposed rocks tend to weather around a “tough” center, leaving rounded cores.
well that’s just great. one of the tectonic plates has lost it’s bearings.
So do Indy 500 drivers. When they get out onto the marbles, at any rate.
When I was a kid I remember reading in an old book that a lot of erosaion was being caused to those old sandstone rocks by wind-driven sand. But when I lived out west, i read angry statements by some that wind erosion was mostly a myth, and that it was occsional water erosion. Sorta SandyHook’s “river that isn’;t there anymore”, but you have to realize that this might mean a temporary fiumara or seasonal flooding, not just a long-vanished river. I suspect this is probably the case – mainly fast erosion by flowing water, aided and abetted by some wind-borne sand abrasion.
I have to add that I was amazed at the rocks in Bryce (the national parks in Utah all seem to be made of sansdstone, but fro different formations and abrading in different ways), and there the forms are very different from the hard but smooth formations in Zion or Arches or Dead Horse Point – it’s made of incredibly crumbly stuff that will come off if you run a finger over it. Bryce gets snow, and I’ll bet that the water-snow-freezing-ice cracking the rock type of erosion plays a major role there.
Its possible these rocks are ventifacts, eroded over aeons by wind and sand. I have a nice example in my cabinet not far from this very chair. Somewhat exotic and worth recognising.
I think a lot of you are confusing rounding with general erosion. There’s lots of erosion mechanisms out there, but (at least inland) there’s not really anything other than a perennial river that will preferentially erode cobble-sized rocks into a round shape.
A typical desert stream where there may be low or no flow most of the year but occasional high-energy flood events is very effective at moving and breaking down sediments, but for various hydrodynamic reasons tend to leave much more angular deposits. Wind erosion can erode a larger rock over time, but because it doesn’t transport them at all, it also tends not to round them. In fact the ventifacts Ken001 mentions are a great example of how it sometimes does the opposite of rounding-- they’re rocks that have become more angular as the windward side preferentially erodes, creating bizzare shapes.
There could be other mechanisms that transported the river cobbles to the OP’s location, but if they are in situ, they’re almost certainly the result of a perennial stream that existed during wetter climatic conditions.
I don’t know much about the geological history of the southwest, but it seems clear that there were many ancient waterways that affected the landscape. Much of the terrain must have been cut away by flowing water over the millenia and those round rocks would have been formed or deposited by those ancient rivers. There may be some altitude where no such rocks are found on the mountainsides because they were never below the water level. Much of what we know about geological history is based on the erosion of terrain by water. As GreasyJack points out, rounded rocks are formed from rivers, and like every other case where the signs of rivers flowing over long periods of time are found, there was a river there at one time.
Locally (Cornwall UK) all the rocks are jagged and volcanic but for some parts of the far South west where the boulders are rounded and smooth the result of a “raised beech” they were rolled round by a huge beech but now are a hundred feet above the beech, the result of the land bobbing back up after the ice sheet receded and the land being freed from its burden.
Those rocks were rolled.
How were those rocks rounded?
By the running of ragged rascals.
Those must be amazing trees. 
More likely to have been oaks IMO …