This natural monument consists of three islands of Plateliai Lake: Briedsalė, Veršiai, Castle, located in Plateliai Landscape Reserve. Briedsalė (Island of Mooses) is the largest island of Plateliai Lake, occupying an area of 5.54 ha in the northern part of the lake. The name of the island comes from the fact that moose liked to go here. The island has flat shores, most of which are occupied by a swamp, dominated by linden and oak. The great crested grebes and terns breed on the island. Veršiai (Calf Island) is an island in the central part of the lake, near the Castle Island, with an area of 2.19 ha. The name of the island is associated with a former tradition where local farmers moved calves to graze throughout the summer. It is overgrown with a mixed forest of natural origin: oaks, linden, spruce and other trees. Their average age is about 100 years old. In the spring, hollow squirrels that are the plants listed in the Red Book, bloom on the island. Castle Island is the most famous island of Plateliai Lake, located in the central part of the lake. It dates back to the 15th century. The castle owned by Lithuanian nobles stood in 1941, but it has not survived. The island was connected with the Šventorkalnis peninsula and its former settlement by a bridge. The poles of this bridge (XVI-XVII century) are protected as a cultural heritage asset. The area of the island is 5.29 ha. The forest growing here is of natural origin, dominated by linden and oak, some of them - more than 100 years old. The only place where the true scabies grow in Lithuania is here. The island is also protected as a cultural monument.