From Fossilworks. Locations around the world where middle Miocene fossils have been found.
There was a worldwide increase in rainfall during the middle Miocene. The authors tell of “global” “strong precipitation” and a “precipitation maximum”. Here is a graphic:
Oxygen levels were also higher. Quote: “Figure 4 shows that there has been a significant reduction in atmospheric oxygen, chiefly since the middle Miocene…” Note: the left side of this graph is the more recent. The Carbon Cycle and Atmospheric CO2: Natural Variations, Archean to Present
Avocados figure large in Opalescence as they did in prehistoric California. “Unfortunately, the more fragile flowers and fruits have not been preserved, hence were lost during the formation of the fossil bearing soils.” Prehistoric Avocados in California
Here is an image from the above site of fossil avocado locations in California.
Volcanos are also (in the book) and were (in the Miocene) a big part. Below is a graphic of the locations of middle Miocene volcanic hotspots (red triangles). Locations and Ages of Middle Tertiary Volcanic Centers in Coastal California
Another interesting occurrence was the presence of a North American honey bee that just happened to live during the middle Miocene, Apis nearctica, the only native honey bee ever discovered from this continent. Here is a fossil impression:
Of course such a worthy animal is included as well!
Yet another item of interest to add to the color and variety of life here in California during the mid-Miocene: there were flamingos!
Here is a photo of a flamingo egg dated 19 million years ago, from San Bernardino, California (other flamingo bones have been found as well).
The International Chronostratigraphic Chart. The arrow points to the approximate temporal location of Julie and Tom in the Miocene.