boulder, large rock fragment formed by detachment from its parent consolidated rock by weathering and erosion. In engineering and geology, especially in the United States, the term is applied to loose rocks having specific sizes according to various systems of classification, i.e. the Wentworth scale (for C. K. Wentworth, American geologist), where a boulder has one linear dimension of at least 10.1 in. (25.4 cm). Boulders transported by glacial ice are usually referred to as glacial erratics; glacial boulder fields, or felsenmeer, containing large blocks of angular rock broken from intense frost action are evidence that the region has experienced glacial action in the past. See drift; moraine.