Late Pleistocene tills containing appreciable amounts of ultramafic clasts and matrix occur on and adjacent to ultramafic bodies (commonly harzburgite) in the northern Puget Sound region. Many exposures of such tills are strongly cemented, typically by aragonite, although Mg-calcite and calcite also occur; cement comprises 1 to 6% of the till.

Cementation appears to be controlled by ground water enriched in calcium derived from calcium-bearing pyroxene in the ultramafic rocks. The role of incompletely serpentinized rocks and ground water is indicated by the following observations. (1) Cementation occurs only in association with incompletely serpentinized rocks; other calcium sources (e.g., limestone) are absent. (2) Calcium-rich cemented-till halos surround fresh ultramafic clasts that were glacially transported from source outcrops; completely serpentinized clasts have no cemented halo. (3) Massively cemented till forms preferentially at topographic lows on ultramafic bodies where ground-water discharge is concentrated. At these sites the till is strongly cemented in the wetted zone at and near the bed-rock contact, and th degree of cementation decreases a few meters above the contact.

These tills are unusual in that they contain the first reported CaCO₃ cement in till that is not derived from preexisting carbonate materials, although the process of carbonate (ophicalcite) generation from unserpentinized ultramafic rocks has been increasingly recognized in recent years.

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