As we find ourselves in a geological epoch of our own making, it becomes necessary to reconsider the temporal scale of ethnographic enquiry; the effect of human behaviour is shown as a mark in deep time. Focusing on the East Anglian fenland, UK, this article considers the importance of thinking about long-term environmental change for the understanding of human life. First, the article explores the way in which human geological agency has transformed the landscape. It then goes on to argue that while the scale of such changes can only be understood against the backdrop of geological time, social life in the region nevertheless demonstrates ‘temporal lock-in’, which is defined in the article as an increasing fixation with the landscape of a single point in history. The consequence of such temporal lock-in is that long-term environmental variability becomes, literally, unthinkable; yet surface-level certainties of the present are called into question when the timescale of deep history is brought into view.