sentences:  Riots swept the free states in waves, usually in urban areas where there had been recent immigration of blacks from the South.The height of these riots was in 1819, with 25 riots recorded, resulting in many injuries and fatalities, although riots continued up through the 1830s (see anti-abolitionism in the North).The back-to-Africa movement was seen as the solution to these problems by both groups, with more support from the white population than the black population.