the knife-edge arête of crib goch (foreground) , pyramidal peak of snowdon (background) both result of glaciation.
the rocks today make snowdon , neighbouring mountains formed in ordovician period. @ time, of modern-day wales near edge of avalonia, submerged beneath ancient iapetus ocean. in soudleyan stage (458 to 457 million years ago) of caradoc age, volcanic caldera formed, , produced ash flows of rhyolitic tuff, formed deposits 500 metres (1,600 ft) thick. current summit near northern edge of ancient caldera; caldera s full extent unclear, extended far summit of moel hebog in south-west.
snowdon , surrounding peaks have been described true examples of alpine topography . summits of snowdon , garnedd ugain surrounded cwms, rounded valleys scooped out glaciation. erosion glaciers in adjacent cwms caused characteristic arêtes of crib goch, crib y ddysgl , y lliwedd, , pyramidal peak of snowdon itself. other glacial landforms can seen around snowdon include roches moutonnées, glacial erratics , moraines.
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