The term "foul line" typically refers to a line in sports, particularly in bowling or basketball, and does not fit the geographical features described in your question. However, based on the descriptions you provided, it seems you are looking for definitions of different geographical features:
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A tall steep area where rivers form rapids and waterfalls - This typically describes a canyon or a gorge.
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A flat lowland area on the coast of the Atlantic Ocean - This could refer to a coastal plain.
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A series of mountains and mountain ranges that are related - This is referred to as a mountain range.
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A raised area in the foothills of the Appalachian mountains - This could describe a plateau or hill.
If you have a specific context or need further clarifications about any of these terms, please let me know!