The Vosek NNM can be found 0.5km north of the town of Rokycany in the Plzeňský kraj – Plzeň Region and lies in the agricultural landscape between the town of Rokycany and the municipalities of Osek and Volduch. The D5 Prague-Plzeň motorway passes through the southern part of the territory. The monument covers an area of 74 hectares, lies at elevations of 378-409 metres above sea level and was declared in 1989. The main subject of the protection is the siliceous concretions of the Šárka and Dobrotiv Formations belonging to the Central Bohemian Ordovicum.
The protected area lies in the north-eastern tip of the Plzeňská pahorkatina Hilly Lands and in the Rokycanská kotlina Basin. The surrounding areas have a forest-free and mildly wavy relief and are intensively farmed. The terrain in the actual protected area is enriched by the elongated Hůrka hill (406.7m) in the northern section.
Concretions from the Šárka Formation are abundant in some of the concretions contain rich fossil communities of Lower Ordovician fauna of trilobytes, gastropods, brachiopods, clams, ostracodes and hyolites, while starfish, Conularia, cephalopods and graptolites are rarer. All of these fossilized animals can be found inside the so-called “Rokycany Balls”. The most well known fossils include the trilobites Placoparia barrandei, Trinucleoides reussi and Ectillaenus kotzeri, the cephalopod Batmoceras complexum, the brittle star Eophiura bohemica, the starfish Mitrocystites mitra, the brachiopod Euorthisina moesta and the molluscs Sinuites sowerbyi and Archinocella ovata.
Concretions from the Dobrotiv Formation are much rarer, less compacted and palaeontologically very poor. During palaeontological inventory research at Vosek NNM in 1992 a total of 120 fossil species were recorded at the locality. The area to the east of the Rokycany – Volduchy road provided the richest finds of siliceous concretions and the most notable finds have been made in this part of the protected area in recent years. The majority of this section is now occupied by the D5 motorway.
The territory of the monument belongs to the Central Bohemian Upper Paleozoic. The geological structure includes formations from the Later Proterozoic, Cambrian, Ordovician, Tertiary and Quaternary periods. The rocky basement is built of rocks of the Klabava Formation and Šárka Formation complexes. Black slates of the Dobrotivské Strata were found in boreholes in the south-western part of the protected area (between the Rokycany – Osek and Rokycany – Volduchy roads). These rock samples included typical fossils for the Dobrotivské Strata, such as abundant examples of the brachiopod Benignites primulus (Barr.) and the trilobyte Cyclopyge bohemica. Large numbers of ball-jointed basalt rocks (diabases) can be found in the ploughed fields on Hůrka hill.
Due to the intensive agricultural character of the territory of Vosek NNM no interesting plant or animal species have been recorded here. The main danger for this locality is not the intensive agricultural usage but the damage caused by amateur fossil collectors when they remove many fossils from their context.