Doug alerted me to an exposure of white rock on Jumbo Mountain south of Darrington. his exposure is from a rock slide in Squires Creek.
The rock slide happens to be located within the Darrington-Devils Mountain Fault Zone. The fault is a major structure and is thought to be associated with a former tectonic boundary between accreted terrains.
From Sauk Quadrangle Geology Map (Tabor and others, 2002)
TKegb: Eastern Melange Belt gabbro
TKhm: Helena-Haystack Melange peridotite and serpentine
TKeu: Eastern Melange Belt ultramafite
The slide may provide some excellent fresh exposures of some rocks that could use a bit more study.
The TKhm are odd slivers of serpentine and peridotite that are likely faulted slivers within much younger sandstone associated with the fault zone. These slivers have been deserpentinized. Vance and Dungan (1977) describe deserpentinization taking lace during a preEocene or early Eocene event, prior to tectonic emplacement of the ultramafic rocks as faulted slivers within the sandstone. Just how these rocks were heated and deserpentinized is an interesting question and the answer has implications on the interpretation of the history of the formation of the Helena-Haystack Melange.