A tertiary alcohol is not readily oxidised because the carbon carrying the hydroxyl group

Explanation

Oxidation of an alcohol involves removing the hydroxyl hydrogen together with a hydrogen from the same carbon, and in a tertiary alcohol that carbon holds three alkyl groups and no hydrogen at all. It can only be oxidised under conditions vigorous enough to break carbon to carbon bonds. This difference is the basis of using acidified dichromate to distinguish the three classes of alcohol.