Slack is a kind of coal - the cheap gritty stuff. If it was in lumps - mainly - then it would be 'nutty slack'. As in 'the coalman only left half a ton of nutty slack this week'.

Newcastle relies on these holes in the ground left by open cast working to avoid doing any civic recycling. Part of Newcastle itself was strip mined about five years ago - it's not often you see that. Opencast inside a city boundary.

A correction to my former comments: I've met Matthew and Matt Snr. On their estate is a teashop, which is associated with their dairy - which makes excellent local cheeses. I took a fieldtrip, from the local agricultural college, to the cafe, for a mid-morning break, (between Oxbow lakes and Upland lakes). They wouldn't come out - I can remember that quite clearly !