Florissant - Fossil Ecosystems of North America: A Guide to the Sites and their Extraordinary Biotas - page 199

the present Mt Princeton in the Collegiate
Range ( 220 ).
Some two million years after the Wall
Mountain Tuff, a new eruption affected
Florissant from an area known as the
Guffey volcanic center, about 25-30 km
(15-20 miles) south-west of Florissant.
This volcanic complex (also known as the
Thirtynine-mile Volcanic Field, 220 )
produced a variety of rock types, resulting
from lava flows, pyroclastic flows, ash
falls, debris avalanches, and lahars
(debris flows containing much water and

mud). The eruptions and the resulting
landscape would have been very similar to
Mt St. Helens in its 1980 eruption ( 221 ),
during which a debris avalanche swept at
27 m.s -1 (60 mph) through the nearby
Spirit Lake ( 222 ) and overtopped
Johnston Ridge; a pyroclastic flow spread
at more than 300 m.s -1 (650 mph),
destroying 600 square km (230 square
miles) of forest; and lahars, fuelled
by melted glacier ice, flowing at up to
12 m.s -1 (27 mph) swept down the Toutle
River and into the Columbia River which,

220

106°W 105°W

Florissant
Fossil Beds
National
Monument

SOUTH
PARK

Grizzly Peak
caldera

39°N

Buena
Vista

Thirty-nine
Mile volcanic
field

Colorado
Springs

Pikes
Peak Mt Aetna

caldera

N

10 km

Bonanza
caldera

SAN
LUIS
VALLEY

10 miles

Miocene sediments Mt Princeton batholith

caldera

Eocene-Oligocene volcanics Paleozoic-middle Eocene sediments

limit of Wall
Mountain tuff

late Eocene sediments Precambrian

220 Geological map of the Colorado Rockies in the vicinity of Florissant Fossil Beds National
Monument, showing relationship of Tertiary volcanic centers, Thirty-Nine Mile Volcanic Field,
and limit of Wall Mountain Tuff to the Florissant area after Evanoff et al., 2001.