Showing counties by their date of formation. In a few cases I used 'settled' dates. For San Francisco I used the date of the establishment of the Mission. The data is from Wikipedia. I grabbed the list of counties, then grabbed the wikipedia page for each county and pulled out the 'Founded' or 'Established' or whatever early date.

(And an update with Rivers and a different color scheme. It is interesting to see how County lines follow, or not, the rivers).