Sign about the Rattlesnake Formation at Mascall Formation Overlook.
Marker Name: A Volcanic Inferno
Marker Text: If you were to remove the single rimrock layer that caps the flat-top mesa directly ahead, you would reveal the surface of an ancient valley bottom.
Imagine standing in that valley just over seven million years ago. Few trees are visible. A lush blanket of grasses covers the length of the valley, swaying wave-like in a light breeze. Nearby, a small herd of four-tusked elephants grazes playfully, ignoring a passing hyaena hunting prey. Suddenly, a distant, thundering explosion shakes the land. A flock of birds bursts from the grasses into the sky.
Perhaps an hour later, the eastern end of the valley quickly fills with a tidalwave of fiery volcanic gases and particles. This onrushing cloud of death flows down the valley at high speed, engulfing and incinerating all life.
Successive ashfalls from the same volcanic eruption, over 80 miles to the south, inundated that ancient valley. These fiery deposits settled to valley bottom, welded, and cooled into a glassy ignimbrite layer - the horizontal layer you now see as the mesa’s rimrock today.
1. Rattlesnake Formation
Hundreds of feet of coarse, sedimentary deposits (fanglomerates), above and below the ignimbrite layer, make up this fossil-bearing formation. Plant and animal fossils found in these layers suggest a grassland environment with few trees.
2. Mascall Formation
After a series of lava flows covered the region (note the basalt layers of Picture Gorge, to the right), ashfalls from the Strawberry volcanics inundated the land. The buff-colored exposures before you are part of this formation. They contain plant and animal fossils that suggest vast forest and grassland savannahs once existed here.
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