פירוש על קידושין 121:5

If the parts of the field that cannot be sown don’t count in measuring the field, then why just ten handbreadths high or low? Even higher ravines or lower rocks can’t be sown.
The answer is that when these features are less than ten handbreadths, they count as part of the field because it is normal for fields to have small rises and inclines. Only if they are ten handbreadths high do they not count.