Marsum (V Diff, 3 pitches, 105 m) is the easiest of six recorded climbs on the series of slabs known as the Whaleback Buttress, situated on the east facing cliffs of Cwm Cau. The slabs are located conveniently close to the path which circumnavigates Llyn Cau, making the climb a handy warm up route for anyone planning to attempt some of the harder climbs the rock face has to offer. Although it’s often used as a precursor to the area’s harder routes, it’s an enjoyable line in its own right, and offers good, clean rock, that dries quickly, in a location that is normally sheltered from the prevailing westerly winds.
The approach begins at the main carpark (SH 732 115), and the start of the Minffordd Path on the southern side of Cadair Idris. The carpark is situated a few metres along the B4405 near the junction with the A487T. If coming from the north you will need to gain the A487T at the Cross Foxes Inn Junction (SH 766 167) on the A470T. If coming from the south you will need to travel north along the A487T from Machynlleth (SH 745 008). Cross the Afon Dyfi just to the north of the town (SH 744 019). Immediately after the bridge the road turns right very sharply, follow the road around along the winding and wooded pas through the villages of Corris and Corris Uchaf. As you exit the pass Cadair Idris will rear up in front of you and the junction off the main road will soon be signposted. The carpark is also serviced by a public bus which runs between Aberystwyth (SN 583 815) and Dolgellau (SH 728 177). Be aware though the bus drivers will drop you off but don't always pick you up afterwards. In these situations you may be forced to hitch, as the next bus will be a long time coming.
The approach takes the Minffordd Path from the main carpark (SH 732 115) on the south side of the mountain to Llyn Cau (SH 715 123). Start by leaving the carpark through the kissing gate at the far right hand (if your facing north) corner of the carpark, just behind the toilet block. Follow the broad gravel path north crossing a small concrete bridge, after around 100m you will reach another gate. Go through the gate and carry on following the gravel path past the CCW run visitors centre on the right, past a ruined farmstead on the left, until you reach another gate (SH 728 115). Go through the gate and begin the steep walk through the oak woodland up into Cwm Cau. From here on the stream that flows from Llyn Cau will always be to your right hand side. The path is generally good, and where erosion is at its worst, wooden steps and gangways have been constructed. After passing through a gate in a stone wall you will exit the woods, although the carry on a little further on the other side of the stream, and the path will take you in an ark into Cwm Cau proper. From here the path is of a more gradual nature, and should be obvious in all but the worst weather conditions. Stay on the path, ignoring the junction that bears left onto Craig Cwm Amarch, and make your way gradually to Llyn Cau (SH 715 123), which is dammed by an obvious moraine. Follow the moraine and the lake shore north, and cross the stream that exits the lake. The slabs are located on the far shore and barring all but the worst weather, should be obvious to anyone who has studied the photo's on this page. Follow the northern shoreline around the lake until you reach the base of Whale-back Buttress (SH 711 123) and the start of the climb.
Start at the left hand edge of the slabs, just below a small groove.
Pitch 1 (30m): Climb through the clumps of heather to gain the groove under which you start, and climb up onto the crag's main slab. Climb up the slab, heading in a leftwards direction, and then move right blow a steeper section to the left end of the terrace. Belay from here or slightly higher up at the top of a short chimney.
Pitch 2 (25m): From the top of the short corner above the stance, move left again, and follow a broad rib above an imposing block at the top of the main slab.
Pitch 3 (50m): Continue to climb up the disjointed ribs above the huge block. Stay on the rock to avoid the vegetation and prolong the climb.
Descend to the base of the crag via a grassy gully that runs parallel on the southern side of the slabs. This can be slippery in just climbing shoes so take care.
A single 60m rope will be fine, a moderate rack of nuts and hexes, and a few slings and screwgates. The route can be done in mountain boots or rock shoes, although boots are probably more advantageous considering the broken nature of the boot. Naturally you will also need all the other paraphernalia associated with hill walking in Wales: full waterproofs, fleece, hat, gloves and of course some lunch.
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OS 1:25k Explorer Series OL 23 Cadair Idris & Bala Lake / Llyn Tegid
OS 1:50k Landranger Series 124 Porthmadog & Dolgellau
OS Road Map 9 Wales/Cymru & West Midlands
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