The topography of a hill provides ideal growing conditions for wine grapes. The altitude of a hill can protect grapes against sudden frosts which occur in a valley, and which could kill an entire crop. Also, the slope provides for good drainage of both air and water down the hill, and prevents excess moisture from accumulating as in the valley below. In the northern hemisphere, southward-facing hills receive maximum light and warmth. For this reason, most vineyards are located on the south end of a hill.
Several Oregon wineries have planted test vineyards in valley floors, and time will tell how these locales compare to their hilly counterparts.