The geological formations of Mesa Verde National Park were mainly deposited between 100 to 75 million years ago, when the Western Interior Seaway covered the middle of the continent. During this time, the sea often advanced and retreated, depositing different types of sediment each time. About 100 million years ago, when the sea first reached the Mesa Verde area, it depostied layers of sand. Million of years later, this sediment was compacted and cemented into the Dakota Sandstone Formation which today forms the erosion resistant base beneath the park and Montezuma Valley to the north.

The sea advanced farther westward about 90 million years ago, and the sandy tidal deposits changed to deep water shale deposits. The Mancos Shale consists of fine particles and organic material. Fossils found in this formation include oysters, clams, shark teeth, and ammonites. The gray shale forms the low hills you see at the base of the mesa in the Montezuma Valley. Places to view Mancos Shale: This formation is best seen from Highway 160 or at the lowest portion of the entrance road.

Point Lookout Sandstone, deposited as beach sand as the sea retreated again, is mainly composed of tan to buff colored sandstone with shale lenses interspersed throughout. Few fossils are found in this formation. Small alcoves form in this sandstone layer, but these were not often used by the Ancestral Puebloans. Places to view Point Lookout Formation: Point Lookout and the Knife Edge are two geologic features capped by Point Lookout Sandstone that can be seen from the entrance road.

After depositing Point Lookout Sandstone, the sea completely withdrew from the area, leaving a flat, coastal plain. Swamps developed as small, slow-flowing streams wound their way to the sea in the northeast. As plant and organic material decayed and accumulated, dark, fine-grained shales formed. Thin beds of sandstones and coal seams can be found within this formation. Leaf impressions, tree branches, and other fossilized plant remains are located in the shales. Places to view Menefee Formation: The Menefee Formation is best viewed from the Geologic Overlook. It is the layer directly above the Point Lookout Sandstone.

The Cliff House Sandstone formation was named for the Ancestral Puebloan homes built in the alcoves formed within it. The sea advanced yet again and the influx of sand generated the thick, orange-buff colored sandstone. Very few fossils are found in this formation due to tremendous wave and biological action in the beach environment. The wave action also generated ripple marks throughout this layer. Places to view Cliff House Sandstone: This formation can be seen from any cliff dwelling viewpoint.

Geologic Column of Mesa Verde National Park
Time		Rock Units
Era	Millions of Years Ago	Group	Formation
Cretaceous	75	Mesa Verde	Cliff House Sandstone
	78		Menefee
	81		Point Lookout
	86		Mancos Shale
	93 100		Dakota Sandstone

What is a mesa? The word mesa is Spanish for "table." A mesa is a raised landform that is flat on the top, and has steep or sloping sides. A plateau is similar, but much larger.

Where did the Ancestral Puebloans get their water? Rainfall and melting snow soak into and down through the porous layers of sandstone until the water reaches an impermeable layer of shale. The process is similar to placing a sponge (sandstone) on a table (shale) and pouring water on it. The sponge absorbs the water; but once it is full, the remaining water seeps through the sponge and pools on the table. So, prevented from moving down through the shale, water makes its way sideways through the rock. It eventually seeps out of the cliff wall and pools to form a small spring. Seep springs are often found at the base of the Cliff House Sandstone and Point Lookout Sandstone formations. The springs provided a source of fresh water for the Ancestral Puebloans.

How do alcoves form? The seep springs created areas where water accumulated for thousands of years. Over time, the continuing action of seeping water, freezing and thawing, wind and rain, broke away the rock and formed alcoves. Some alcoves became large enough to hold cliff dwellings.

Why are they called alcoves, not caves? Caves are underground chambers; alcoves are arched openings within a wall.

(Information courtesy of the National Park Service)