Geology 150U/390T Field trips, June 1/2, 2002 Page 3

Three hours west of Denver on I-70 is the Dotsero exit. A frontage road runs east from the exit on the south side of the freeway and nearby is the youngest volcano and lava flow in Colorado dated about 4,140 years old by carbon dating of wood beneath the flow.

Here we are exploring the scoria and basalt which erupted from the Dotsero cone less than a mile to the north-east.

Beyond the flow a short distance is the Eagle River which was dammed for a time by this flow.

Lava flow near Dotsero, CO, just south of I-70 exit 133. Looking SE.

	South of the area above (towards the river), the original lava flow began cooling and the leading edge of the flow slowed down. Lava began "piling up" and internal pressures caused it to squeeze up into mounds. There are several such "squeeze ups" in the area towards the river. Note that some of the mounds in the background are simply bulldozed piles of rock on private property.
Squeeze Up in lava flow. Looking south towards Eagle River.

Looking north at the Dotsero cinder cone. The cone is the dark colored "mound" between and beyond the two hills.

This area, like many other volcanic locations in Colorado, has been extensively mined and quarried. The cone and immediate surrounding area now bear little resemblence to the original landscape. For instance, there is little evidence of the lava flow coming down the ravine between the hills; it's all been hauled away. There are also major mining scars on the cone itself.

Volcanic cinders (scoria) from the cone are crushed and used in making certain type bricks; the origin of "cinder blocks".

Dotsero Cinder Cone from the frontage road.

	A road cut exposes some of the Flat Top lava flow. The basalt here is in an area highly enriched in gypsum and it contains large amounts of sulphate zeolites.
About 8 miles north of Wolcott, CO on highway 131, looking west.

Continuing north on 131 towards McCoy, another road cut exposes sedimentary rock containing many fossils. Here an intrepid explorer braves the loose shale to get a closer look.

Note the steeply inclined sediment layers; evidence of massive tectonic forces at work. The folding of rock and sediment layers in this area is attributed to "salt tectonics". Large bodies of salts deposited by the evaporation of ancient seas and lakes are less dense than surrounding rocks and tend to "flow" under the weight of overlying material. The inexorable movement of large salt bodies cause local rock layers to distort leading to the almost vertical orientation in the sediment layers seen here.

Sedimentary deposits exposed by road cut on highway 131 near McCoy, CO.

	Finger Rock; an impressive volcanic neck. Necks are the exposed and eroded remnants of the solidified magma tube that fed a volcanic vent; the "fossilized plumbing system" of an ancient volcano. Finger Rock is dated to be 7-10 million years old.
Finger Rock. Highway 131 south of Yampa, CO.