What are lengthy and narrow ridges of glacial gravels referred to as?

Youre probably talking about an esker, which is a deposit of material (not necessarily just gravel) that appears as a sinuous ridge, typically ranging between 10 and 50' in elevation. Eskers are formed in meltwater tunnels underneath glaciers. The water deposits the gravels and sands (etc) in the tunnel, and when the glacier melts from around the tunnel, it appears as a ridge on the landscape.





Below is a great photo of an esker.

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