The reserve comprises the south-western part of Stora Stensholmen with the small islands Korpholmen and Björkgrunden. The islands are of moraine, with parts of them being rocky ground. Stora Stensholmen is characterised by old coniferous forest that has been relatively unaffected by forestry.
The impression given by the island is of natural forest. Traces of old meadowland can be found on the southern part of the island, and there are plans to restore the meadowland here. Both Korpholmen and Björkgrunden are covered by forest, and the islands have a rich animal life. Stensholmsfladen is popular with boat owners seeking outdoor recreation.