Here is the reason I believe it was a threat and post 647 was used as plausible deniability, making anons think it is about [D]ay of days. Three materials were used: shells, rocks, slate. Here are the attributes of each:

Tan 8 Shells sand = Tonight send shells

Rocks 6 = Rock the 6 O'clock

47 slate = Slated for President Trump

Tonight send shells and rock President Trump's 6 o'clock.

Beach walk (thinking about being a bitch while walking)

Formation = A formation can be a form or something to be formed in the future.