while your context is about olive trees, walnut wood is particularly famous for its grain which goes "all over the place" and is much sought after for furniture because of the decorative effect. Besides a walnut wardrobe I have a craftsman-made wizard's staff out of a branch of a walnut tree and some of the knots have made "faces" in the wood. Given the author's love of personalizing natural things I would not be surprised if he had this in mind.

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Note added at 9 hrs (2012-06-02 07:17:01 GMT)
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another possibility:

"twisting their thick bark into fantastic shapes" since it's actually the bark that's twisted.

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Note added at 9 hrs (2012-06-02 07:25:26 GMT)
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about the "tacto": I can't think of a way to include this in English but the author has an evocative way of using the reader's senses to make him or her "be there" in that place of time. Here he is using the sense of touch to make you reach out with your mind to touch the bark and feel its roughness.

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Note added at 9 hrs (2012-06-02 07:40:17 GMT)
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a shorter way to say it would be:

"gnarling the thick trunk"

gnarl
tr.v. gnarled, gnarl·ing, gnarls
To make knotted; twist.
http://www.thefreedictionary.com/gnarl