Read the entire blog post for more details. I clicked on the link to the Atlas of Historical County Boundaries (http://publications.newberry.org/ahcbp/) and saw:
The site does not permit clicking on a state in the map area. You have to pick from the dropdown menu on the right, or pick from the list of states, to pick a state.
I picked California from the "Interactive Map" dropdown menu, and saw:
I changed the date to 18 February 1900, and selected all but the "Unsuccessful Proposals" layer to be visible, and only "Modern County Seats" to be "active," and saw:
By clicking on the "Chronologies" link at the bottom, the user can see a list of all of the county historical boundary changes described by date, additions to and subtractions from which other counties, etc., plus a reference for each change. Here's the California list:
As on most websites, the Help page is your friend. For this site, reading the Help page will shortcut your trial and error searches on this site. The Help page is here:
I haven't explored some of the more complex features of the site. I see that, in the "Downbload GIS files" on the home page, that the user can download files for each state in GIS, KMZ (Google Earth) or PDF formats.