Complex sentence: In the Wachtendonckse Psalmen this feature is very rare, while much later it can be seen in the spelling of Dutch toponyms which indicate the sound change was taking place during the 10th and 11th century.
Split into simpler sentences: In the Wachtendonckse Psalmen, it is very rare, but much later, it can be seen in the spelling of Dutch toponyms. Thus, the sound change was taking place during the 10th and 11th century.