There are other meanings that don't require the carving to be a form of writing: https://en.wiktionary.org/wiki/geoglyph

The proper glyph concept pertains to something cut into something or out of something as background; it is essentially of a 3-d phenomenon and makes no sense apart from it; the concept has nothing whatever to do with writing, which is almost never like that. A piece of lead type is indeed a glyph, and this is how you use it to print, but the printed text has no glyphs.

They're geoglyphs, which are symbols encoded in the earth.