Most of the sand and gravel deposits of the Missoula Valley are composed of plutonic igneous and high-grade metamorphic rock, for the most part non reactive as aggregate for use in concrete mixes. The sole exception to this is the hypabyssal volcanic rock that is found in the Pattee Canyon Alluvium. Sand and gravel recovered from the gravel-rich glacial ice-raft pockets of the nearby north hills of Missoula are sometimes found with pockets of rock-flour silt delivered by ice rafts. Such materials, even with washing, are sometimes inadequate for use in making durable concrete.