I'll tell a story

A long time ago during an ice age, so much water was frozen over land that the ocean was two hundred feet lower and the Sacramento River was a fast rushing river carrying rocks down from the mountains, grinding them, smoothing them, and rounding them into cobbles. Later when the weather warmed, the ice melted and the ocean level rose back into San Fransisco Bay, backing up the Sacramento, slowing it down and creating a slow moving estuary to fill in with mud. It is a bit complicated but this is something to be understood: ice age means low sea level which means fast river and rounded cobbles being formed; warm period means high sea level which means slow river and cobbles silted over with mud.

They say this cycle has happened three times in the last 100 thousand years. One ice age is said to be about 25 thousand years ago, another 60 thousand years ago, and I don't know about before that. That is three separate times when cobbles were formed.

If you visit Silicon Valley today, you can see it is an estuary where the Sacramento flows into San Fransisco Bay. The cobbles buried in the mud are still there, more than 10 feet below the surface of the dirt. Local landscapers dig them out and use them as decorative borders around shrubs and trees, in parking lots and around hotels. The rounded cobbles are very pretty, like something you expect to see in a Japanese Garden.
That is the background I understood when I was in Silicon Valley and I found stone tools made out of the cobbles and mixed in with them. Those tools were probably made during an ice age. That is not too surprising, that people were in California making stone tools about 25 thousand years ago. The first things I found looked pretty "fresh" and were all of a certain style. But there were other heavier axes in there too, of a different style that were a bit rounded and smoothed - part way back to being cobbles. If you think about it, this means they were part of the cobble production of the last ice age, which means they were tools made during a previous ice age - around 60 thousand years ago. That is a bit more surprising.

So that's my story: stone tools made from glacial cobbles which themselves got turned back into cobbles. It means there were people in California making stone tools two ice ages ago.