https://en.wikipedia.org/wiki/Mercia#/media/File:Mercian_Supremacy_x_4_alt.png
This map appears to be quite 'specific' as if the border may follow a river or some such. But it's north of the Mersey and as far as I can tell north of the Alt, too. At the far western end of the northern border, north of Liverpool, the border pretty much bisects where I grew up, Formby. For that 'local interest' reason I suppose, and because I am very interested in 'Dark Age' history, I would like to know in which Anglo-Saxon Kingdom the area I grew up would have been in.
It seems from the maps I've seen to be very much a border country between Mercia and Northumbria. The Battle of Chester, for instance, was won by a Northumbrian king and that was very much south of the Mersey. The Northumbrian heartlands appear to be in the North East, the Mercian heartlands in the Midlands. It leaves me wondering what was the greatest cultural influence and longest lasting control over what is now Merseyside. Or was it simply not very settled, perhaps? Maybe it was just useless bog/fen?
The place is also awash with Viking place names (including Formby) but I'm talking about specifically before Viking settlement.
Even before the heptarchy when there were other smaller kingdoms, it's unclear to me where the area that is now Formby point would have 'been'. Perhaps the answer is simply "it was contested/border country between Northumbria and Mercia" but more detail would be nice. To return to the picture I linked, also, I'm very curious what the border is 'following' exactly, if that can be ascertained.
Well the boarder of the kingdom of Mercia did change a number of times throughout its existence but i believe that map is depicting it during the point its border was marked by the river Ribble where other points it would have been around the Mersey.
There were large sections of that area that was a forested at that point, mixed in with more populated farming land and villages, while not uninhabited it certainly wasn't as important a region as it is today. One thing to bare in mind though is that it takes a little time to raise an army and as such an invading force might well be able to make its way a little into the territory of a rival before a large enough force to face them in an open battle could be assembled.
As for the culture of the region the kingdoms of this time weren't really nations in any modern sense. Its hard to be certain how much it meant to the average person that they were Northumbrian or Mercian but its likely even in times were the borders between these kingdoms were well established those cultural differences that existed between such kingdoms were more gradients than they had hard straight lines with the land between the Mersey and the Ribble being something of a frontier region. Certainly my the time Mercia had become the most powerful kingdom in England overtaking Northumbria celtic influence was rare in this region but very early on in Mercian history these regions would have been on the edge of both the inflluence of the Welsh kingdoms to the west and the Hen Ogledd kingdoms further north who were both Brittonic dialect/language speaking broad and disunited groups. However the Hen Ogledd, Cumbric speakers were largely driven further north and welsh further west out of the region over time with any who remained under Anglo-Saxon rule being assimilated.
Concerning before the Heptarchy you're likely to get even less clear answers, the exact boarders of the early Anglo-Saxon kingdoms and the Brittonic kingdoms of the the British Isle are unclear for the most part today and may have been fuzzy zone of control at the time to.
