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Spring
2015
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Rocks in Rocklin

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by Richard Hilton, Earth Science Professor

Richard HiltonRocklin California is located between Roseville and Auburn along Interstate 80. Rocklin is well known for its rocks. The most obvious rocks seen from here are granites. Granites to most of us are not that exciting because much of the Sierra Nevada mountain range is composed of them. However, finding granitic rocks almost on the valley floor, close to a population center, and right next to not only a major highway but also a transcontinental railroad was a significant find. These granites have been quarried for building stone and monuments since the 1800s. 

From a distance, granite looks light gray or white in color. However, if you examine it closely, you will see that it is composed of small, white, gray, black, and sometimes pink colored crystals. The light colored crystals contain silica and oxygen. These are known as silicates. Minerals in this group include quartz (gray or white), and feldspars (pink or white). The dark crystals are composed of iron ("ferrum" in Latin) and magnesium in addition to the silica and oxygen. These are known as the ferromagnesian silicates. Minerals in this group are usually black in color and include hornblende and biotite mica.

The granite formed from a mass of hot, molten rock called magma. The magma cooled and crystallized miles below the surface of the earth 140 million years ago. From that time on, the crust containing the granite was uplifted and is still rising today. Over millions of years the miles of rock that had been overlying the granite was eroded away. The eroded material was deposited in the Great Valley. These sediments make up the rich agricultural soils of the Sacramento and San Joaquin Valleys.

Between Rocklin and Folsom Lake there are outcrops of sedimentary rocks that were deposited on the exposed granite. The numerous fossil shells within the rocks reveal that about 70 million years ago the area of Rocklin was under water and near the oceans edge. Besides fossil seashells, evidence of fish, sharks and marine reptiles have also been found. These sedimentary rocks are collectively known as the Chico Formation. Plant fossils also found in this formation indicate that Redwoods, cycads and ginkgoes thrived in the Sierra at that time.

Sand and gravel cemented together into a rock called conglomerate appear as outcrops in low-lying areas. These ancient river gravels are mainly composed of rounded quartz and could contain gold. The gold mined during the 1800s in the local creeks probably originated from these deposits. The gravels were deposited in riverbeds approximately 50 million years ago. At this time, rivers flowed from what is now central Nevada to the seashore near Rocklin. Locally, these sands and gravels are rich in fossil wood and leaves. The fossils show us that the climate was warmer and moister than our present climate. These deposits are part of what is called the Ione Formation named after the town of Ione, California. Here, as in Lincoln, a delta of a large ancient river emptied into the sea and deposits of sand and clay as well as plant material occurred. These ancient deposits provide sand, clay and coal for local industry.

On the ridges around Rocklin and also deposited on top of the granites are younger volcanic rocks contained in a matrix of fine sand and mud. The larger volcanic clasts come in two forms, rounded and angular. Rivers that use to flow across the area ten million years ago transported and rounded the volcanic rocks. These rocks originated from the Tahoe area where active volcanoes were once common.

The angular rocks are contained in lava mudflows called lahars. Similar to the eruption of Mt. Saint Helens, some of the Sierra's ancient volcanoes produced vast amounts of ash. This occasionally would mix with water or melting snow to become mud. The mud flowed over the landscape collecting lava rocks and other material as it flowed. When the flow encountered a river, it took the path of least resistance and followed the course of the river all the way to the Roseville area.

Besides the angular volcanic rocks, almost anything in the path of these mudflows was incorporated in to the flow. Rocks, soil, leaves, pinecones, and tree trunks were carried along. Fine examples of some of the fossil plants are displayed in the Sierra College Natural History Museum.

Since the time of the mudflows, the Sierra Nevada has uplifted and tilted to the west. This caused new rivers and creeks to cut deeply into the Sierra's flanks leaving these ancient rivers choked with volcanic mudflows suspended as weather resistant caps on ridge tops. Over time and as the lahars weather, the finer matrix material is transported away leaving the landscape littered with the larger resistant lava rocks. 

Rocks such as granite, conglomerates, sandstones, claystones and lahars are common in the Rocklin Area. They reflect a rich geologic history, and are literally the foundation for our modern history.

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