Geologic Formations

Almost the entire memorial is in an area known as the Montezuma caldera. The complex nature of the geology is mainly a result of this cataclysmic eruption and later volcanic events. There are areas of relatively young Cretaceous sedimentary rock overlain by older Jurassic volcaniclastic conglomerate, which is overlain by still older Pennsylvanian-Permian limestone. This juxtaposition of age is further compounded by Tertiary intrusive rhyolite, dacite, and diabase and Holocene surficial units.

The northeastern part of the Memorial is dominated by large blocks of interbedded sandstone and red shale and blocks of calc-silicate hornfels overthrust on younger porphyritic granite.

The northwestern and north-central parts of the memorial are dominated by nearly vertical cliffs and escarpments of granite and, at the highest elevations, dacite tuff. The dacite tuff along the ridge summit was probably laid down as an ash flow during the initial eruption of the Montezuma caldera. The granite was intruded later, after the eruption of the caldera. As this molten rock worked its way upward, it ripped blocks of Paleozoic limestone from the surrounding rock. Many of these large blocks are visible immediately around the Visitor Center and Coronado Cave.

Much of the southwestern part of the memorial is dominated by volcaniclastic conglomerage. This material caps most of the area from Montezuma Pass and Coronado Peak eastward along the ridge. In many places the conglomerate is dominated by limestone clasts. In other areas the limestone is subordinate to sedimentary and volcanic clasts.

Below the conglomerate to the north and east is a complex of volcanic tuff, lava, limestone, sandstone, and shale. The complexity of this area is the result of a pair of faults that run directly up Montezuma Creek and a series of faults in the south-central part of the memorial. These faults altered many of the rocks in the area as well as the geologic uniformity of the area.

The southeastern quarter of the memorial is dominated by Quaternary terrace gravel deposits and alluvium derived primarily from granite with a few small isolated areas of exposed limestone and granite bedrock. Most of this area is a stable, gently sloping fan terrace dissected by nearly level ephemeral drainageways.

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