There are several valuable forms of nature in the Warta Mouth Landscape Park such as the Postomia Valley nature reserve which is situated between Słońsk and Lemierzyce, along the winding river Postomia. The Reserve’s area is 68.65 hectares, covering the picturesque southern slope of the Toruń- Eberswalde Urstromtal, the bed and the riverside reeds of Postomia between Lemierzyce and Słońsk. The bottom of the valley is occupied by rushes and meadows. The reserve is a sanctuary of old trees, more than 80 of which qualify for protection as natural monuments. There are oaks, beeches, hornbeams, elms, poplars and maples. The thickest oak has the circumference of over 600 cm, while many other oaks and beeches exceed 400 cm in circumference.
The nature reserve is located 2 km from the center of Słońsk, by the road to Głuchowo. The convenient entrance is on the right side of the road, a few meters before the bridge on Postomia– the only one between Słońsk and Głuchowo. There is a clear dirt road leading to the reserve which is a good point to start exploring, especially as tourist information boards will accompany the educational path.