Maybe this is explained later, and if so, please don't mention the resolution. However, if it's never mentioned again, I'd like to hear some reasoning.
Basically, Strider leads the hobbits and that mistreated pony to a place where it seems like they (or he) had agreed to meet with Gandalf. Although I can't remember reading that, even though I've been so careful and really tried to follow every detail.
Anyway, let's assume that they did arrange to meet Gandalf there. Even if so, it strikes me as bizarre that Strider would notice and turn around a random little rock in that area, find some minor scratches and immediately assume that they mean:
Gandalf was here at the third day of this month.
It's clear that he just guesses and that they didn't have some kind of secret agreement to communicate using special rocks which they place in a specific way and use a secret code language. He just draws this conclusion based on... something?
If it indeed turns out that it was Gandalf who etched those lines into the bottom of that rock, and if it really meant what Strider guessed it means, that almost stretches the story beyond my suspension of disbelief even in spite of all the supernatural elements. It just seems too strange, unless they indeed have such a "system" agreed upon between them two.
Maybe I'm being too impatient, and should finish the books before I draw any conclusion about this, but it makes me feel uncomfortable due to how far-fetched it seems.