The Mt Ventoux of Wales, if not the UK! Taken from the south this is one of the classiest, hardest hills around. Steep for a long time at around 14% and then, near the top it ramps up to nearly 30% for a short section when you are at your limit. There is absolutely no chance to recover anywhere on the climb. The road surface is also a bit off-putting, being both gravely and steeply cambered. It is also narrow, so there is little room for weaving, and great big ditches either side of the road ready to swallow you and your bike if you weaken! All these factors ramp up the difficulty, if not the actual gradient. It is certainly very hard if you’re not mentally prepared, or if you don’t have low enough gearing.
A more amenable circuit can be done by starting in Bala then cycling down the southern side of Llyn Tegid to reach the col from the north. The return journey takes you down to Llyn Efyrnwy - here you have the choice of looping the lake or just cutting up to the north end of the lake. Either way, you then climb back over Bwlch yr Hwch and sweep down Cwm Hirnant to Bala.
At 546m Bwlch y Groes is the highest road col in Wales; consequently during the winter months it is occasionally blocked with snow.
There are various starting points, depending on your intended route, but Dolgellau or Bala are obvious choices.