Agreed. And it was the volume grain production which supported the villa system. But if we accept that volume grain production is just one economic model, rather than being the only one, the fact hat such a system did not eixst in the north doesn't automatically mean that the north was economically inactive or underdeveloped.
But not impossible. Cattle bones found at the forts at Carlisle and Birdoswald show the same mutations, suggesting a shared and local supply.
Again, I stress, lancashire, south of the Vale of Eden. Roughly between the Mersey and River Lune. I think with the Lune and Ribble flood plains and the other numbers smaller streams feeding the area with water, it was too low, too wet and the soils too heavy. To the north however, as you can see from this relief map, Carlisle and the Wall are well served by better soils. If you know of a map of british settlements in lancs, I'd be grateful. It is supposed to have been a territory of the Brigantes but, during the roman period, their important places were to the east at places like Stanwick Castle and Aldeborough.