Yorkshire. Other summits towards the northern end are Hay Bluff (Welsh: Penybegwn), , Rhos Dirion, and Lord Hereford's Knob or Twmpa, . Towards the south of the range are the more independent summits of Crug Mawr at , Pen Allt-mawr at and the peak of Pen Cerrig-calch which rises prominently above Crickhowell in the Usk Valley. Outlying summits, all of which are classed as Marilyns, include the Sugar Loaf (Welsh: Pen-y-Fal), Mynydd Troed and Mynydd Llangorse. The lower and separate hills of Allt yr Esgair, Myarth, Bryn Arw and Ysgyryd Fawr (also known as 'The Skirrid', Skyrrid or 'Holy Mountain') are scattered along the southern fringe of the Black Mountains. In his work People of the Black Mountains, Raymond Williams described the Black Mountains thus:See this layered sandstone in the short mountain grass. Place your right hand on it, palm downward. See where the summer sun rises and where it stands at noon. Direct your index finger midway between them. Spread your fingers, not widely. You now hold this place in your hand. The six rivers rise in the plateau towards your wrist. The first river, now called Mynwy, flows at the outside edge of your thumb. The second river, now called Olchon, flows between your thumb and the first finger, to join the Mynwy at the top of your thumb. The third river, now called Honddu, flows between your first and second fingers and then curves to join the Mynwy. The fourth river, now called Grwyne Fawr, flows between your second and third fingers and then curves the other way, south, to join the fifth river, now called Grwyne Fechan, that has been flowing between your third and your outside finger. The sixth river, now called Rhiangoll, flows at the edge of your outside finger. This is the hand of the Black Mountains, the shape first learned. Your thumb is Crib y Gath. Your first finger is Curum and Hateral. Your second finger is Ffawyddog, with Tal y Cefn and Bal Mawr at its knuckles. Your third finger is Gadair Fawr. Your outside finger is Allt Mawr, from Llysiau to Cerrig Calch and its nail is Crug Hywel. On the high plateau of the back of your hand are Twyn y Llech and Twmpa, Rhos Dirion, Waun Fach and Y Das. You hold their shapes and their names. Geology Bedrock The Black Mountains are composed almost exclusively of rocks