Page Type Page Type: Route
Location Lat/Lon: 52.69310°N / 3.90774°W
Additional Information Route Type: Trad Climbing
Seasons Season: Spring, Summer, Fall, Winter
Additional Information Time Required: Half a day
Additional Information Difficulty: V Diff (USA 5.2-5.4)
Additional Information Number of Pitches: 3
Sign the Climber's Log


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.

F.A: B. J. Marsden and G. Summers, October 27th 1973.

There were signs that it had been climbed before.

Getting There

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.

Cwm Cau from the Minffordd Path. Tapia Llwydion is the series of slabby crags on the right hand side of the panorama, just above Llyn Cau. Photo by Nanuls

Route Description

Marsum Photo-topoPhoto-topo of the route (Photo by Nanuls). For an unsoiled photo click HERE!


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 Natural Resources Wales 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.

The Route

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.

Essential Gear

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.


Open Space Web-Map builder Code
Navigation Maps

OS 1:25k Explorer Series OL 23 Cadair Idris & Bala Lake / Llyn Tegid
OS 1:50k Landranger Series 124 Porthmadog & Dolgellau

Road Maps

OS Road Map 9 Wales/Cymru & West Midlands


Snowdonia (Official National Park Guide) Snowdonia (Official National Park Guide) by Merfyn Williams with contributions from Ian Mercer and Jeremy Moore

A handy book full of useful information and interesting facts about the National Park.
Climbers Club Guide Wales: Meirionnydd Climbers' Club Guide Wales: Meirionnydd by Martin Crocker, John Sumner, Terry Taylor, Elfyn Jones, with contributions from Mike Rosser, Mike Lewis and Dave Wrennall

The definitive climbing guide to the area. Contains detailed descriptions and excellent diagrams of all known routes on Cadair Idris. An essential purchase if you plan to do a lot of climbing in Mid Wales.

External Links

Yours truely after completing Marsum (V Diff)Nanuls near the end of the route (Photo by JoeHarris)
Joe after completing Marsum (V Diff)
JoeHarris at the end of the route (Photo by Nanuls)
Tal y LlynTal y Llyn (Photo by JoeHarris)

Government Bodies and Official Organisations

Snowdonia National Park Authority

Council for National Parks

Association of National Park Authorities

Natural Resources Wales


Royal Commission on Ancient & Historical Monuments in Wales

Gwynedd Archaeological Trust

Snowdonia Society

The National Trust

Hiking, Climbing and Mountaineering Organisations and Companies

British Mountaineering Council

The Climbers’ Club


Plas y Brenin National Mountain Centre

Hightreck Snowdonia


Mountain Weather Wales

Weather from the Met Office

BBC Weather

Tourist Information

Visit Wales

North Wales Tourism Partnership

Local Information from

Local Information from Snowdonia Wales Net

North Wales Index


Welsh Public Transport Information

UK Train Timetable


Youth Hostel Association in Wales

Pete's Eats

Pen-y-Gwryd Hotel

Maps and Guidebooks

Ordnance Survey

Harvey Map Services

Cicerone Guidebooks

Climbers’ Club Guidebooks


North Wales Bouldering

Cordee Travel and Adventure Sports Bookshop

Wildlife and Conservation

Joint Nature Conservation Committee

Natur Gwynedd

North Wales Wildlife Trust

Royal Society for the Protection of Birds



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