Much of the very prevalent cobbles and boulders occurring as Lake Missoula debris are ellipsoidal in form, probably due to the dominance of Belt Supergroup metasedimentary rock (quartzite and argillite) and glacioIluvial boulders. Both types of which, for quite different reasons, tend to provide enough flattish surface so as to be placeable in walls with a minimum of surface microtopography, thus presenting a more pleasing appearance than is generally possible when using more spheroidal river-run boulders.

Alluvial river boulders can be found in the channel of the Missoula River and more prevalently along the Rattlesnake Canyon drainage, which, surprisingly is the only such canyon to reach up into igneous and metamorphic units which held the most durable rock from canyon sides in locations for which downstream transport eventually reach the City.