On the Old Maps map of the area there's a feature called Black Dyke (rems of) which could be your feature. I suppose you could also think about glacial features, man made rabbit warrens (pillow mounds), a park pale, military practice earthworks, there's also some mention of coal on Old Maps but it doesn't look like a pit heap.
cheers
fitz

There are two Long Cairns on the SMR in Northumberland, I believe, but both have a rounded plan. Please don't say 'there are none'. What you have seems to be an eartwork of some kind. Until you begin to understand the superstitions of the builders you'll not make much progress interpreting the features you discover.

The large stone pictured, with the female figure before it, is an erratic. But it's been stood on end and securely bedded. The carved grooves, at the top, prove that. Therefore it's a Standing Stone - and should be listed here as such. The Park archaeologists - I can dig out there contact details - should be told.

An accurate plan of these sites would be helpful - don't forget to include the extra bits from The Mare and Foal. Discovering (what-seems-to-be) the UK's tallest standing stone is quite an honour. One of the stones in the Langley stone circle is reported to have three boat-shaped cupmarks - a picture of these would be handy ...

Cheers for the thoughts fellas,

I'm thinking pillow mounds could fit the bill, as there are medieval fish pond earthworks not too far away.

If I'm really lucky, I might get allowed out of my box for a few hours tomorrow, and will have the chance to go have another look-see.