The Causey Mounth is an ancient drovers' road over the coastal fringe of the Grampian Mountains in Aberdeenshire, Scotland. This route was developed as the main highway between Stonehaven and Aberdeen around the 12th century AD and it continued to function as the principal route connecting these two cities until the mid 20th century, when modern highway construction of the A90 road occurred in this area. There are extant paved and usable sections of this road over part of the alignment; however, many parts of the ancient route are no more than footpaths, and in some cases the road has vanished into agricultural fields. Constructed in the Middle Ages, the Causey Mounth was created as an elevated rock causeway to span many of the boggy areas such as the Portlethen Moss. A considerable portion of the alignment of the Causey Mounth is illustrated on the UK Ordnance Survey Map, although a large fraction of the route cannot be navigated by a conventional passenger vehicle (particularly at the crossing of the Burn of Pheppie).
The Mounth is the range of hills on the southern edge of Strathdee in northeast Scotland. It was usually referred to with the article, i.e. "the Mounth". The name is a corruption of the Scottish Gaelic monadh which in turn is akin to the Welsh mynydd, and may be of Pictish origin. Traditionally the people of northern Scotland did not consider the Grampians to be a single range, but several, and these were known as the Mounth or the Mounths. Mountain ranges to the west are known as Monadh Liath and the Monadh Ruadh, which could be glossed as the Grey Mounth and the Red Mounth.
The mountainous landforms of the Mounth form an outlying ridge of the Grampians stretching from Ballater in the west to the North Sea coast immediately north of Stonehaven. Historically they formed a formidable barrier which, to some extent, isolated the northeast of Scotland from the Scottish Lowlands, physically and culturally. In the Middle Ages an ancient roadway known as the Causey Mounth was built to connect Stonehaven to Aberdeen using an elevated rock causeway design to penetrate this boggy area of the eastern Mounth. This route was by way of Cowie Castle, Muchalls Castle, Portlethen Moss and the Bridge of Dee. The route was that taken by Earl Marischal and the Marquess of Montrose when they led a Covenanter army of 9000 men in the first battle of the Wars of the Three Kingdoms in 1639.