It was declared a National Monument under old NMC legislation on 22 September 1978.
Further Reading
https://www.google.com/search?q=Zonnebloem+colledge&rlz=1C1SQJL_enZA866ZA866&oq=Zonnebloem+colledge&aqs=chrome..69i57j46i433j46j46i175i199j0l3.16284j0j15&sourceid=chrome&ie=UTF-8