I prefer the singular, but if your author wants the plural, that's OK too. But you need to add something to indicate the altitude.

http://en.wikipedia.org/wiki/Moorland
Moorland or moor is a type of habitat found in upland areas in temperate grasslands, savannas, and shrublands and montane grasslands and shrublands biomes, characterised by low-growing vegetation on acidic soils. Moorland nowadays generally means uncultivated hill land (such as Dartmoor in South West England), but the Old English mōr also refers to low-lying wetlands (such as Sedgemoor, also SW England). It is closely related to heath although experts disagree on precisely what distinguishes the types of vegetation. Generally, moor refers to highland, high rainfall zones, whereas heath refers to lowland zones which are more likely to be the result of human activity.[1]

Examples of 'plateau grassland':

Late Quaternary eolian dust in surficial deposits of a ...
pubs.er.usgs.gov/publication/70028522
United States Geological Survey
Mar 12, 2012 - Late Quaternary eolian dust in surficial deposits of a **Colorado Plateau grassland**: Controls on distribution and ecologic effects.

www.jstor.org/stable/1934461
JSTOR
by PC Lemon - ‎1968 - ‎Cited by 24 - ‎Related articles
EFFECTS OF FIRE ON AN **AFRICAN PLATEAU GRASSLAND**. PAUL C. LEMON. Department of Biological Sciences, State University of New York at Albany, ...

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Note added at 5 hrs (2015-02-25 04:16:04 GMT)
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The point here is that plateaus/mesas are often arid, so the author is emphasizing the grassy aspect.