I thought about something like that when looking at the lofty mountain map, and then I interprete them as

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such a high wall to climb up!

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such a deep hole on my way... Is there a bridge nearby to go on the other side ?
In other term, the darker area is more easily read as a hole than as an obstacle.
But on the other hand, it looks to me that you already have some material for building a cliff that would fit nicely with your tileset:

(okay, it requires more tweaking that my quickly-fried edit, clearly)