I think you are describing the acidic tuff-turbidite bands that crop out at the opencast workings up at the top there - stripey rocks a bit like liquorice allsorts. They are of considerable geological interest because they have undergone brittle deformation in the slate-forming event - the muds were squished into slate but these rocks cracked. The fissures were mineralised at the time - and some of the minerals present offer potential for U-Pb isotopic dating, which has interesting scientific possibilities. Similar bands are present in the modern Cwt y Bugail quarry and both are broadly correlated with the Rhiw-bach Volcanics.