These two mounds next to the River Swale, close to the village of Grinton, are probably glacial moraines. The eastern (left) mound has a clear embankment around it. The western mound was partially dug out to make a Royal Observer Corp monitoring station during the 'cold' war. This made a small reinforced concrete room which is now empty and sealed.

The whole site is on private land and there is no access.

The locally known name is Grinton How. How means 'hill' and there are several 'hows' in the area.
The name 'Ox Hill' appears on the 1857 OS map, but is not used locally.

The Brigantes Nation web-site refers to it as Grinton Fort and suggests this site may of Roman origin, though 'unknown' may be a better choice. This web-site has many photographs, a plan and details of the site.

The ADS web-site suggests it is medieval.

The Out of Oblivion website says it's probably an iron age defended hilltop enclosure. This site has a very good aerial photograph link.

English Heritage Pastscape reference linked from the ADS site says medieval has been suggested.

The 1857 OS map marks it as 'remains of a camp, supposed Roman.
This map is linked from the Pastscape site above.

The Gatehouse web-site refers to it as Ox Hill, which is the 1857 map name, and also suggests it's medieval with a motte and bailey fortification.

Basically no-one knows!

It was a Royal Observer Corp monitoring post from 1965 to 1991. It is referred to here. and here.

A description of a different ROC post is given here.

Royal Observer Corp underground monitoring posts were used to record blast strengths and radiation levels in the event of a nuclear attack.