At the beginning of the period the Time of Troubles disorganized the frontier defenses and led to much destruction. The Oka region was raided almost every year, but with decreasing frequency. 1633 may have been the last crossing of the Oka. We hear more frequently of raids at such places as Kursk and Voronezh, implying stronger defense in the north and a southern movement of the Russian population. The Belgorod Line of forts was begun about 1633 and seems to have had noticeable effects after about 1640. We hear of raids on Poland in 1614-21, 1624-28, 1633, 1640 and 1642.. The Belgorod Line of forts had a noticeable effect for how many years?