Chamosite ooliths from the Winter Gill ironstone (Bajocian) possess a distinctive internal structure and oblate ellipsoidal shape which are believed to be due to the following factors: 1. Accretion occurred under unidirectional current conditions on a surface of previously formed chamosite mud. 2. Ooliths formed around nuclei which were fixed by partial burial in the mud, as well as around free nuclei. 3. The oblate ellipsoidal shape is related to lateral accretion of suspended chamosite particles on stationary ooliths; fixation of the particles may have been influenced by organic films. 4. The presence of a distinct trend, with increasing size, in the envelope structure of each oolith is related to a) an upward limit to available suspended material, and b) to changing conditions resulting from intermittent movement of ooliths towards the site of accumulation. The chamosite mud is thought to have been formed on an upstanding sand-bank, probably by alteration of detrital clay. The present ironstone was formed by a continuous process of formation, transport, and accumulation of ooliths together with slow erosion of the chamosite mud surface. The ironstone is situated against one flank of the sand-bank and passes laterally into sandstone.