Bava Batra 103 - Selling a Tract of Land
A beit kor is a tract of land on which a kor of barley seed can be sown. It is fixed at 75,000 square amot, or between 4 to 7½ acres.
If one says to his fellow, "I am selling you a beit kor of earth - that is, land suitable for cultivation - and there were clefts 10 hand-breadths or more in depth, or rock 10 hand-breadths high, their are is not included while measuring the area of the sold land. Instead, the seller must provide land of level ground, to the full measure of beit kor . Clefts smaller than 10 hand-breadths, however, do contribute to the total area.
If the seller said, "I am selling you about a beit kor ," then even clefts larger than 10 hand-breadths count as part of the area, because "about" means in whatever condition it is.
Art: Georges Seurat - Alfalfa Fields, Saint-Denis