A Geological Tour of Denver, Golden, and Colorado's Front Range

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The Idaho Springs Batholith

Injection gneiss along the Highway 40/Interstate 70 corridor

Bands of injection gneiss along the Highway 40/Interstate 70 corridor, between Golden and Morrison.

A Geological Tour of Denver, Golden, and Colorado's Front Range

By Jack Barkstrom

The formation known as the Idaho Springs batholith, making up the foothills west of Golden and Morrison, can be as much as 4,000 feet thick in places.  It is very old, having formed between 1.7 and 1.8 billion years ago.  Much of it is gneiss or various stages and forms of granite. 

Granite can be composed of a number of minerals, although the most common components are feldspar, quartz, and mica, or hornblende.  Feldspar, quartz, and mica, are, in turn, silicates, combinations of the element silicon and oxygen.  Feldspar is an aluminum silicate, formed from soda, potash, or lime. Gneiss, composed of the same minerals as granite, is a form of granitic rock, formally differentiated from granite, more by pattern or texture, than by composition.  It is characterized by parallel banding, known as schistosity (highly developed in rocks classified as schists) or foliation.  The banding can range in size from barely visible parallel streaks to bands many feet or yards in width, illustrated by the pegamatitic intrusions, known as injection gneiss.

Along the Highway 40/Interstate 70 corridor, south of Golden, the alternating bands of orange and black rock give the appearance of a multicolored waterfall.  They range in size from a few inches to fifteen or twenty feet across.  Within these bands or the boulders and rocks scattered nearby can be seen the more conventional gneiss patterns which distinguish them from granite.