"line in the sand"

[Jesse Sheidlower]

This doesn't seem to have been discussed on ADS-L. I wonder
if anyone has anything useful on the figurative use of _line
in the sand_ 'a boundary; limit of tolerance' etc. The earliest
example we have is only from 1978, which seems quite late, but
all the earlier cites I've laboriously tracked down have been
literal.

Anyone have something earlier, or a suggestion of why such a
seemingly predictable phrase would be so recent?

Thanks.

Jesse Sheidlower
OED