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July 29, 2009 > Unearthing the Ice Age of Irvington

Unearthing the Ice Age of Irvington

By Joyce Blueford

The Tri City area has many natural geological wonders that the Math Science Nucleus, a non profit organization, is bringing to the attention of the community through the Children's Natural History Museum. This series of articles will allow the reader to understand their worldwide significance. If interested in helping to develop a comprehensive concept plan please contact the author (

Part 1 - A Geologic Bonanza

When the dinosaurs ruled the world, the Tri City area was covered with deep water as the Pacific Plate slowly moved toward the North American Plate. Silica rich biological ooze was forming in deep water containing a microscope organism called radiolarians that are now exposed in rocks at Coyote Hills and Morrison Canyon. Long after the dinosaurs became extinct the San Andreas Fault system evolved stretching from southern to northern California. In our area the stress of the moving plates caused the Calaveras Fault to break, followed by the Hayward Fault. Slowly the land began to emerge as the landscape changed from marine conditions to a vast low lying coastal area where large herds of mammals would evolve and dominate the San Francisco Bay area.

An extraordinary story began to unfold within our local area during the beginning of the Ice Age about 1.8 million years ago. In the Irvington and Mission Districts of Fremont, Sabercat Creek and Mammoth Creek were probably one large braided stream. They meandered down an incised valley caused by newly forming mountains to the east. The surrounding stream was lush with native riparian plants like buckeyes, sycamores, oaks, tupelos, and bay laurel as well as aquatic plants like tules and cattails. Large mammals - mammoths, sabertooth cats, camels, horses, giant short-faced bear, giant ground sloths, and mastodons - ruled the area. This fauna lasted until 300,000 years ago roaming on a savannah-like landscape that extended for miles into the present San Francisco Bay and Pacific Ocean.

The Hayward and Calaveras Fault systems slowly created the southern Diablo Range which prevented the flow of water through this area and slowly choked a once mighty river. The land corridor was reduced and the climate changed which altered the food chain. Local habitat could no longer support these animals. While the smaller animals thrived, many larger mammals became extinct. Remains of these creatures, however, can still be found in the conglomerates, sandstones, and siltstones that once were the river's sediment.

As humans inhabited the area maybe as early as 11,500 years ago, most of the larger mammals of the Ice Age were already extinct. The period around 4,000 years ago must have been a time of immigration throughout California. Early inhabitants (i.e., Ohlones) managed the land's productivity through fire and trading with other tribes. Europeans and the later American settlers from the East are credited with the introduction of many non-native plants and over-hunting of large mammals resulted in changing the landscape even more.

Some steep valleys and areas within Fremont have escaped development and represent a time period of about 300 years ago. These corridors preserve surviving animals from the Pleistocene such as tree frogs, pond turtles, mule deer, ground squirrels, wood rats, valley pocket gophers, cottontail rabbits, red foxes, badgers, striped skunks, and mountain lions. The Sabercat Creek Corridor is one of these areas. As streams erode rock, Irvingtonian Gravel is exposed, revealing fossils of the early Pleistocene.

Finding, excavation, and later re-burial of these fossils by a freeway are an incredible and unique local story of efforts by scientists and amateur paleontologists to understand these Pleistocene animals.

The next article of this series will look at how these fossils were found and how one person helped to save them so we can learn about and explore the world of these extinct creatures.

Children's Natural History Museum
4074 Eggers Drive, Fremont
(510) 790-6284

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