Hi Rune, here are the links for the Towthorpe Plantation barrows info at English Heritage, hope they are useful to you. I've just had a quick flick through a few of them and it looks like they may all have been constructed using some quantity of Duggleby clay although there is a suggestion that the reddening of the clay could have occurred as an interaction with ash within the barrow.

http://www.magic.gov.uk/rsm/26534.pdf

http://www.magic.gov.uk/rsm/26535.pdf

http://www.magic.gov.uk/rsm/26536.pdf

http://www.magic.gov.uk/rsm/26537.pdf

http://www.magic.gov.uk/rsm/26538.pdf

http://www.magic.gov.uk/rsm/26539.pdf

http://www.magic.gov.uk/rsm/26540.pdf

If you haven't already got them then I'd recommend the Godfrey Edition old OS maps 1905 for the North and South Wolds (sheets 64 and 72), they're only a couple of quid each and show the area from the north of Driffield to the south of Beverley and it looks like 100 years ago the area was still peppered with barrows.
I'll have a dig round and see if I've got any more recent pictures of the sites around that area.

Cheers-
Chris