I'm not a geologist; did I misunderstand the Wikipedia entry I linked? It says

- "Because of their high density, monazite minerals concentrate in alluvial sands when released by the weathering of pegmatites. These so-called placer deposits are often beach or fossil beach sands..."

And I found two specific examples of notably radioactive monazite beaches—an 800 km stretch of Brazil's coast [0], and 55 km stretch of India's coast [1].

[0] https://en.wikipedia.org/wiki/Guarapari#Radioactivity

[1] https://www.thehindu.com/sci-tech/No-major-birth-defects-fou...