Types of topogenous fen include basin and floodplain fen, whilst types of soligenous fen include mires associated with springs, sills and flushes in the uplands, valley mires, springs and flushes in lowlands, track ways and ladder fens in blanket bogs and laggs of raised bogs. Fens can also be 'poor' or 'rich'. 'Poor fens' are derived from base-poor rock such as sandstone and occur mainly in upland areas. Short vegetation (usually with a high proportion of Sphagnum species is characteristic for this type of bog, as is acid water. 'Rich fens' are confined to lowland areas and areas where there are base-rich rocks (e.g. limestone) and are usually very diverse. Large swathes of fen are uncommon in intensively farmed lowland areas, and fens in this area tend to be smaller and more fragmented. Exceptions to this include the Insh Marshes in Scotland (covering an area of 300ha), the fens of Broadland (covering 3000ha) and the Lough Erne system in Northern Ireland.