Park. As an image, the millstone is widely visible on literature but use is made of the objects themselves at many of the entrances to the National Park. These rocks extend northwards through the South Pennines of Lancashire and West Yorkshire and westwards into the Forest of Rossendale and West Pennines and the Forest of Bowland, also in Lancashire. At the Yorkshire Dales they cover the south-east edge, but north of the Craven Faults, due to weathering, they form only cappings to separate hills. Northeast Wales A small area of Millstone Grit Group rocks stretches through Flintshire and Wrexham into the northwest corner of Shropshire near Oswestry. South Wales The term "Millstone Grit" was also adopted in South Wales where rocks of similar age and lithology are found though the Millstone Grit Series of this region has recently been formally renamed by the British Geological Survey as the Marros Group. The thickest bed of sandstone within it was known as the Basal Grit and this has now been renamed as the Twrch Sandstone. The Farewell Rock was formerly considered to be the uppermost unit of the Millstone Grit series of South Wales though it is now included within the overlying South Wales Coal Measures. Ireland The term has also been adopted at Slieve Anierin in northwest Ireland, describing the series of shales, grits, and coal seams, occurring from the base of the Namurian upwards. Origins The Millstone Grit dates to the Namurian Stage of the Carboniferous Period. At this time a series of isolated uplands existed across the British Isles region. One particular east-west aligned landmass stretched from Wales through the English Midlands and East Anglia to the continent and is now known as the Wales-Brabant High, though was formerly referred to as St George's Land. Other uplands the erosion of which would provide the source material for the Millstone Grit lay to the north and northeast of the region. The Pennine Basin received input of sand and mud largely from southerly directed rivers from these northern landmasses. Rivers running north off the Wales-Brabant High deposited material in the southern parts of the Pennine basin from northeast Wales to the Peak District. Southerly flowing rivers from this same landmass were responsible for the Millstone Grit/Marros Group succession in South Wales. During much of the Carboniferous Period, world sea-levels were fluctuating in response to the growth and decline of a series