Jamieson, Fausset and Brown on Genesis 29:3

3. thither were all the flocks gathered; and a stone, &c.--In Arabia, owing to the shifting sands and in other places, owing to the strong evaporation, the mouth of a well is generally covered, especially when it is private property. Over many is laid a broad, thick, flat stone, with a round hole cut in the middle, forming the mouth of the cistern. This hole is covered with a heavy stone which it would require two or three men to roll away. Such was the description of the well at Haran.