The Morrison Formation in a measured drill-core section from Dillon, Colorado, is 226 feet thick and is largely a clastic accumulation of wind-and water-transported altered volcanic dust, clay minerals, and angular to rounded sand-size fragments, mainly quartz. The bottom 60 feet of the formation is a fine-grained siliceous marl containing abundant clay minerals, scattered sand grains, calcareous shell fragments, and chemically precipitated carbonates. Above the marl an interval 110 feet thick includes an alternation of more or less sandy mudstones and calcareous mudstones, and calcareous and noncalcareous sandstones. The top 56 feet is noncalcareous mudstone. Altered volcanic dust is abundant in the upper 117 feet of the formation.
Chemical analyses and petrographic characteristics indicate that a large fraction of the materials in the formation came ultimately from a rhyolitic volcanic source, but sparse to abundant sand grains in many layers indicate intermixing of materials from other sources. Abundant clay minerals in the formation formed prior to, and after deposition. After deposition, illite and cryptocrystalline quartz developed at the expense of originally deposited components.