Military History/Stone cannon ball Washington State
I found a round stone ball, about 5-6 inches in diameter, on a beach in Sekiu Washington. I have found nothing like it before. I do see small stone balls that have taken a round shape do to the waves but this is much, much larger and more precise. Is there any chance that it could be a cannon ball? I do note that they are found on the east coast... Being that this is the upper west coast I question what it is. Thank you for any assistance.
Stone cannon balls were only used in the medieval ages before cannon were cast to specific dimensions, and techniques were discovered to cast balls.
What you probably have is an clast eroded by geological processes.
In rivers with high gradient, like those coming off the mountains of the northwest, the flow regime in high in energy since the angle the water flows in fairly high. Looking at aerial photos of Sekiu on google earth, I see that the beaches appear to have been lined with rip-rap, that is stone brought in to prevent erosion.
I also see that the beach of Sekiu is a very perfect half moon beach, which means when storms come in, a lot of erosion would take place, which is why they placed the protective stone. They "armored" the beaches against the refractive waves caused by the two bounding headlands on either side of the bay.
How the spherical stones formed I can offer a few ideas.
Rounded stones form in high energy streams, where they are bounced along the bottom at times of high flow. Sand moves the same way but at lower flow rates.
Now imagine a big ass flood event where the water is 15-20 feet deep. The rocks that kind of power can move would be bigger than footballs or basketballs. Well all the knocking together, called saltation, abrades and breaks the sharp edges off and we get the nice smooth rounded stones, usually made of granite or very hard sand or siltstone. Other rocks with noticeable cleavage planes will crack and split along those lines.
So how does a rock bet perfectly or nearly perfectly round? Have you ever walked a dry stream bed, and seen pockets in the bed rock that are partially filled with gravel?
Well the pockets are carved into the underlying rock by the gravel being swirled round and round in the pocket by flowing water. Kind of a nature made rock polisher. If a rock gets caught in one of these pockets and stays there long enough, it can become spherical.
Another way is being buried in soil, where the soil acids attack the rock on all sides.
You have seen the rectangular cracks in outcrops right? Well below the ground, water and humic acids in the soil attack those cracks and make the wider, eventually the cracks are very wide and the blocks are now isolated chunks of rock buried in the soil and the acids and moisture continue to attack the blocks making them more and more spherical. Later if the soil erodes, a boulder field of round looking rocks are exposed.
Since the rock was hauled in, it most likely came from glacial till, which is cobble sized rock left over from the last glaciation period which began a million years ago. The mountains of the north west had large glaciers and they spilled out into the flatlands. They covered Vancouver, and the Seattle area filled and carved Puget Sound and its related bays and waterways.