Sidewalk glass grids

https://www.kqed.org/news/11791667/what-are-those-grids-of-glass-in-the-sidewalk-and-why-are-they-purple?fbclid=IwAR20I092Guev1m5HHTOV1Yac91Hy2MKPVYLbNUI36PR5gJAAPMfdisRCx-4

A neat footnote to urban history.

(Thanks for the link, Christy!)