Simplify the following text for a Grade 12 native English-speaking person.
Original: Their further progress north was stopped by the southward movement of the Nilotic tribes , while the Nilotic Kavirondo in their turn had their wanderings arrested by an irruption of Elgumi people ( themselves probably of Nilotic origin ) from the east .
Simplified: The Nilotic Kavirondo in their turn had their wanderings arrested by an irruption of Elgumi people ( themselves probably of Nilotic origin ) from the east .