“(…) Count Adolf started building the castle of Segeberg and fortified it. Because the land was ravaged, he sent messengers to many regions, namely to Flanders, Holland, Utrecht, Westphalia and Frisia, with a message that if they needed more land they were free to come with their families, and they would be given a lot of good fertile land, full of fish and game, rich in vast pastures. He also told Holsatians and Sturmarians: ‘Aren’t you the ones who conquered Slavic land and obtained it through deaths of your brothers and relatives? So why should you settle as the last ones? Be first, start migrating to this desired land, settle it and benefit from its resources’ (…). After that call, huge crowds of people from various nations started coming, together with their families and property, to the land of the Wagrians ruled by Adolf, who promised to give them land. Holsatians were given safe areas to the west of Segeberg, near the Trave river, including the Zuentineveld [modern Bornhöved] fields as well as everything between the river Schwale up to Agrimesov [modern Grimmeisberg] and up to Lake Plön. The region of Dargun [modern Warder and Ahrensbök/Stoob] was settled by Westphalians. The region of Utyn [Eutin] was settled by Hollanders. The region of Süsel [south-east of Eutin] was settled by Frisians. The region of Plön still remained depopulated for the time being. Regions of Starogard [Oldenburg] and Lutjenburg as well as other areas along the sea coast were left for farming to the Slavs, who now had to pay rent to Count Adolf (…)”.
The map attached above shows the ethnicities of North-Central Europe during the 9th century AD. As can be seen a large part of modern Germany was at that time inhabited by Slavic-speaking peoples, who were usually called Wends by their German and Danish neighbours. The area which was already at that time German-speaking is known as Old Germany (Altdeutschland).
Within Altdeutschland, there exist just two main clusters of German people. South-West Germans can easily be distinguished from North-West Germans using PCA. Interestingly, Danish populations overlap with North-West Germans.
What distinguishes Altdeutschland from the Ostdeutschland (East Germany) is that East Germans usually harbor high levels of Slavic or Baltic (in case of East Prussia) ancestry. Slavic-speaking populations living between the Elbe and the Vistula rivers, mixed with – and partially were assimilated by – German settlers during the course of the Late Medieval Ostsiedlung: