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The reservoir is contained behind a rock gravity dam and , at its upper end , it is bounded between Arenig Fawr and Arenig Fach , two of the mountains of south Snowdonia .
 Arenig Fawr, Arenig Fach, Snowdonia
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
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Answer:
 Arenig Fawr, Arenig Fach, Snowdonia