Bielsk is at present a little town situated 17 km. to the north-east of Plock. It was mentioned for the first time in the Mogilno document, bearing the date 1065 but probably written down in 1146, which donated the settlement to the Benedictine order. In connection with the settlement the document mentions the church of St John the Baptist, several inns and a market place from which the Benedictines were entitled to collect fees. Jan Powierski assumed that the donation document had also mentioned a stronghold in Bielsk, but he supposed that the original text had been deformed by copyists and the name had been recorded as Oselzch - Osielsk. He also stressed that in the village or its immediate vicinity no traces of a stronghold had been found. The closest fortified object of that type is situated in the village of Mokrzk, 4.5 km. from Bielsk. There is also an impressive two-part stronghold in the village of Proboszczewice on the river Wierzbica, 7 km. to the west of Bielsk, built at the end of the 10th or beginning of the 11th century at the crossing of important routes going through Old Mazovia. The name of the stronghold on the Wierzbica is not known. There was, however, a notable correlation between the names of early-mediaeval strongholds in Mazovia, containing the suffix ' -sk/-sko' or '-ck/-cko', and the names of the rivers on which they were located. The name of the stronghold was usually derived from the name of the river; for instance the stronghold in Nasielsk was situated on the Nasielna, Plonsk - on the Plonna (Plonka), and Sonsk on the Sona. The stronghold in Proboszczewice lies on the left bank of the Wierzbica, a tributary of the Skrwa, which in the Middle Ages was called Bielica. We can probably assume an analogous correlation between the name of a fortified place and a river: a stronghold on the Bielica should have been called Bielsk (Bielsko). It is unclear whether the stronghold declined in the 11th or 13th c. The old settlement may have been destroyed by one of numerous forays against Mazovia made by Prussians, Jatvings or Lithuanians. With time, a new village, called Proboszczewice, arose close to the old settlement centre on the Bielica, while Bielsk was moved to a new location, 7 km. to the east of the old place. Such translocations of old settlement centres in the Middle Ages are evidenced throughout Mazovia (for instance: Stara (Old) Lomza and Lomza, Stara Warka and Warka, Stary Rypin and Rypin). In 1373 Bielsk received municipal rights. It appears that Proboszczewice may be the place to look for the remains of one of the oldest churches in Mazovia - the church of St John the Baptist, mentioned in the Mogilno document. (4 Figures).
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