Picws Du is formed from the sandstones and mudstones of the Brownstones Formation of the Old Red Sandstone laid down during the Devonian period. Its summit and southern slopes are formed from the hard-wearing sandstones of the overlying Plateau Beds Formation which are of upper/late Devonian age. The northwestern and northeast faces of Picws Du were home to small glaciers during the ice ages which gouged out the cirque of Pwll yr Henllyn and Pant y Bwlch. These cwms drain via the Afon Sawdde into the River Towy to the west. They also drain into the River Usk to the north, which then drains into the Usk Reservoir, visible from the summit of the mountain in the middle distance. The southern slopes drain into the Afon Twrch and the slopes to the east so into the River Tawe. The glacial lake of Llyn y Fan Fach is situated below the summit to the west, and is only slightly smaller than its companion Llyn y Fan Fawr about 2 miles to the east. There are several large linear moraines below the cliffs, similar to that below Fan Hir, and some of which retain small pools.