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Darwin Col(12800ft), Lamarck Col's(12960ft) approach is from North
Lake.
Both are class 2, Darwin having more loose scree in a gully that
heads up the eastern side from Blue Heaven Lake.
Never been over them, but both Echo Col or Wallace Col go from Lake Sabrina over to the JMT. I think both are considered class 2-3, from what I've read extremely rough, and I'm just guessing here but I think they may be more winter than summer routes.