The tills of Holderness may well contain different suites of erratic rocks. Many of the erractics are grey and brown and not very distinctive. Here is a list of the more distinctive ones seen in situ compiled from my own observations, the Lewis Penny Collection and the Yorkshire Type Erratics Collection.

Withernsea Till at Withernsea - Chalk, grey flint, red chalk, coal (common), Middle Jurassic Rootlet Bed, New Red Sandstone, basalt, Old Red Sandstone, Pseudomytilloides dubius.

Skipsea Till at Skipsea - Chalk, grey flint, black flint, Old Red Sandstone, yellow quartz, coal, Gryphaea, dolerite, basalt, Larvikite, gneiss, pink chalk, New Red Sandstone, white quartz, orange sandstone (? Tertiary), Carboniferous Limestone, Jurassic belemnite, Speeton Clay belemnite, porphyry, fossils horsetail (Jurassic), greensand, sandstones, Middle Jurassic plant bed, granites, black shale, quartzite, coarse brown sandstone, amygdaloidal lavas, dolerite, Cheviot porphyry, jasper, dark red microgranite, small modern shells, Brockram, carnelian.

Basement Till - Arctica islandica, Gryphaea, black flint.

Links -
reports of the East Yorkshire Boulder Committee
The Lewis Penny Collection
The Yorkshire Type Erratics Collection