Is the ground dry? Could they be cracks in the soil, or maybe there's been a lot of rain and it's a drainage channel?
I believe moles usually push the soil somewhere, which is what a "mole hill" is.
I've seen rats in compost heaps make channels like that, but that was INSIDE the mound of rotting material.
I would have thought if it was animal related it would need to have been under some sort of cover to protect it from predators. I've had some old sheets of timber lying on the ground in the garden and mice have made smaller channels under the sheets, but I've never seen that in open ground before.
My 2p worth.
Bob