The land surrounding Stirling has been most affected by glacial erosion and deposition. The city itself has grown up around its castle which stands atop an ancient quartz-dolerite sill, known as the Stirling Sill, a major defensive position which was at the lowest crossing point on the River Forth. Stirling stands on the Forth at the point where the river widens and becomes tidal. To the east of the city the Ochil Hills dominate the skyline with the highest peak in the range being Ben Cleuch, although Dumyat is more noticeable from Stirling. The Ochils meet the flat carse (floodplain) of the River Forth to the east of the distinctive geographical feature of Abbey Craig, a crag and tail hill upon which stands the 220 ft (67 m) high National Wallace Monument.