[6] The formation consists principally of tholeiitic olivine basalt flows[1] with interlayered beds of alluvium.

The shields are very gently sloping (less than 1.3 degrees) except for La Segita Peaks, which may be superimposed on an older basaltic andesite volcano.

The shield volcanoes are free of spatter and cinder, indicating extremely fluid lava that erupted nonexplosively.

[7] The unit can be divided into three sets of flows separated by alluvium in the southern San Luis Valley.

[8] In 1971, Burroughs added the Mesita Member, a trachyandesite flow in the San Luis Hills, to the formation.