Powder, its quite likely water soluble

It's a realy interesting tightrope. If you just stand in a field holdong signs your don't really get media attention. in order to get that attention you must do something that grinds peoples' gears enough to have media outlets pay attention to them. But that kind of action needs to skirt the vandalism vector, as otherwise people would be like 'they removed the unimportant turnip of Weddelsex, but I dont care' on the other hand You also cannot be too radical, as it will hurt your cause.

It would be great if enviromentalists had a voice that could be audible over the control over media that is enacted by big companies (murdoch f.i.), but theres little big money in the message of climate awareness, and it's a message most people dont't reallt want to hear.

So... You take aim at objects that are deemed worthwhile and important for the people you wish to reach and try to allign your message with the importance of those ancient and important works.

It's a losing battle as people choose comfort over complicated issues (seemingly) out of their control as annoyance, furthermore being made co-defendant in the case of climate destruction is rather jarring, therefore people are shy to pick up on them, as why should the burden be on them?

So theres no way to positively make your message. Therefore any demonstration is jarring per se, even if peaceful it needs to be at least known, and ironicaaly the best way to do that is to do something outrageaus, as our reptile brain goes very hard on that.

good thing no historic monuments/items were destroyed and your comment is completely off topic.

There's no need for the media to spin anything, the protestors committed vandalism and, unless they are protesting the existence of prehistoric monuments, they did a really shitty job of even calling attention to their cause.