Akmenos "Didžioji rėva" is a natural heritage object protected by Šilalė district municipality. The shoal of the riverbed covers an area of 0.33 ha. A shoal is a rocky river threshold. Geological research has shown that meltin glaciers formed huge valleys in the vicinity of Andriejaitiai and Kilpinė. The riverbed is currently formed on the lowest terrace. The slope of the Akmena River is 1.86 m/km, and the slope from Kilpiniai village reaches even 3 m/km. Therefore, the river wash away the sediments and fine particles of sand and clay with a powerful force. And at the bottom there are stones—boulders. The strait in front of the Great Reef for a long time did not allow the boulders to be washed away. In this way Akmena stacked a stone threshold in this place—"Didžioji rėva".