This is a longer walk from Grasmere than most of the guide books seem to suggest - I would think you're not going to do it in less than about 4-5 hours round trip. The main pathway starts on a regular road, and then onto some well made slate paths take you along the eastern side of the valley and beck until you start climbing up towards the waterfall, and then it gets a bit more tricky - though nothing to bother about unless you don't want to pick your way slowly over boulders. Once at the top, the walk around the tarn itself is more or less flat, and you can then return to Grasmere on the opposite (west) side of the valley). This return route is more challenging: though overall it's probably less steep, it's not so clearly marked and there are many more random boulders to deal with (and I would think definitely easier to come down than it would be to go up). The view from the top, and the changing scenery on the way, are worth it - I would definitely do it again.
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