At NKYViews, we find it curious how Carroll would be formed from Gallatin. What is now Gallatin didn't separate from Carroll; what is now Carroll separated to form a new county. Most counties were established when they wanted to break away from the larger county: Kenton chose to separate from Campbell. Grant chose to separate from Pendleton.
But the folks in what is now Carroll already had the county name - Gallatin - and the county seat, but they chose to jettison what is now Gallatin in order to re-name the county, and make it smaller. There are reports of Gallatin being upset on being made so small, so we're pretty certain the division was not at the request of what is now Gallatin.
What political entity ever elected to cut itself in half?
In 1838, Carroll did. That doesn't make sense to us, and while we're sure there's an explanation, we fear that explanation is lost in time.