Fold; anticline; syncline; fold axis কী?

Example 20.10 (The Varden folds)

The limbs of the Varden folds dip $12{}^{\circ}$, so the cover was shortened by $e = 1 - \cos12{}^{\circ} = 1 - 0.978 = 0.022$, about $2.2\,\%$: a stretch of the region $10\,\mathrm{km}$ wide today was $10/\cos12{}^{\circ} \approx 10.22\,\mathrm{km}$ wide before folding. Measured on section A–B as drawn, the limbs would look about $28{}^{\circ}$ steep, because the vertical scale is stretched 2.5 times: $\tan\theta$ is multiplied by 2.5, and $\arctan(2.5 \tan12{}^{\circ}) \approx 28{}^{\circ}$. Read at face value, that would give a shortening of $12\,\%$, more than five times too much.