The Rare Rhomb Porphyry
Large blocks of rhomb porphyry are quite rare in western Jutland and they are not generally used as ashlar stones in church walls – with a very few exceptions. Fousing Church is remarkable in this context in that it has more than 40 very large ashlar stones of rhomb porphyry in its walls. It is noteworthy that two nearby churches also contain rhomb porphyry ashlar stones; Ølby Church has about 20 that are somewhat smaller than at Fousing, and Gimsing about 10 that are about half the size of those at Fousing. It appears that there was special delivery of rhomb porphyry by ice from Oslo to this area.
Larvikite is a characteristic plutonic igneous rock from the Oslo region. Larvikite is geologically closely related to another characteristic rock type – rhomb porphyry. Larvikite crystallised from molten rock (magma) deep below the surface. But occasionally this magma reached the surface where it emerged as lava from a volcano. This lava solidified to rhomb porphyry. Together with blocks of larvikite, rhomb porphyry was transported by glaciers and deposited in western Jutland. The volcanic rock rhomb porphyry did not survive transport by glaciers as well as blocks of larvikite. Rhomb porphyry is one of the many types of stones that can be found on beaches in the area.