And of course the northern border of Mercia is the Mersey. That's why associating Marple, (which like the rest of Cheshire is really not part of the Northwest, but part of the northern midlands as the Cheshire dialect demonstrates) with Manchester across the Mersey is not logical. While the southern border of the county, with Shropshire is indeed only 1000 or so years old, the northern border is much more ancient.
It can be of course that for purely administrative purposes, part of Cheshire and part of Lancashire are treated as part of the same administrative area (as indeed part of Lancashire was included in the old County Borough of Stockport), but that doesn't change the real borders of the counties and even if we must have the current administrative boundaries, that doesn't stop us recognising the traditional borders for other purposes. As for the changes in 1974; at the time, an official of the Department of the Environment wrote in a letter to the Times concerning the new 'counties'. “They are administrative areas and will not alter the traditional boundaries of counties, nor it is intended that the loyalties of people living in them will change.”
For more about the traditional borders see
http://abcounties.com/There is also
http://county-wise.org.uk/counties/never-abolished/