White clay is generally an earlier deposit, before the gravels, or layer above that has contained an iron occurance to turn it yellow colours upon oxodation. Theres nothing to say that geological movements havnt flipped something along the way, so its still worth testing. You may just hit a harder clay/silt/sandstone at the bottom however. But as far alluvial digging goes, generally, its move on/up time. If theres nothing in the clay that has nice potential, its not worth it. Stick woth the gravels with broken quartz