More than 600 erratic boulders of ironstone were found on the western shores of Beitstadfjorden, inner Trondheimsfjorden, but none on the eastern shores. This suggests the existence of an ironstone formation at the bottom of the fjord, and plant remains indicate a middle Jurassic age. The pure ironstone consists of 70-80% siderite, with the remainder kaolinite. Another type contains more or less clastic grains, mostly of quartz. The petrography suggests that formation took place in a warm climate in a quiet limnic environment with a chemical precipitation of siderite, and a deposition of some kaolinite clay and locally derived quartz.