The City of Exeter lies on the East side of the River Exe on a ridge of land backed by a steep hill. In this point the river opens onto a wide flood plain and estuary. Historically this was the lowest crossing point of the river Exe that used to be navigable up to the city. until the construction of weirs later in its history. This combined with the easily defensible higher ground of the ridge made the current location of the city a natural choice for settlement and trade. In George Oliver's The History of the City of Exeter, it is noted that the most likely reasons for the original settling of what would become modern Exeter was the "fertility of the surrounding countryside" and the area's "beautiful and commanding elevation [and] its rapid and navigable river". Exeter sits predominantly on sandstone and conglomerate ridge which forms the backbone of the city, on which the Rougemont Castle is situated. The Cathedral is placed on the edge of this ridge and is therefore visible for a considerable distance.