150 Boise Icons: Sandstone

awebb@idahostatesman.comApril 18, 2013 

0418 local icon sandstone.JPG

Did you know? If you look closely at sandstone, you can often see telltale signs of its granite origins, including flakes of mica.

ANNA WEBB — awebb@idahostatesman.com

Consider historic structures in Boise and you'll hear three words again and again.

Two are "Tourtellotte" and "Hummell," principals of the architectural firm that arguably influenced the Boise skyline more than anyone, or anything, else.

The third: Sandstone.

It's all around us - in the thick Romanesque walls of St. John's Cathedral, in tombstones in local cemeteries, in foundations of homes in Boise's oldest neighborhoods.

It's everywhere in the surrounding natural world, too. When you walk in the Foothills, many of the prominent land forms you see are sandstone, said Sam Matson, a lecturer in Boise State's Department of Geosciences.

To understand the origins of local sandstone, start 5 million years ago, when the Valley was covered in water.

Lake Idaho - about 100 feet deep at its deepest point, a mud bog at its shallowest - stretched from the Owyhees to present-day Weiser, from the Boise Front on its northern edge to Hagerman in the east.

To get a sense of what that looked like, take a drive up Bogus Basin Road during a heavy inversion. The cloud layer replicates the edge of the lake, with just the Owyhees and the Boise Front visible above the clouds, said Matson.

The mountainous shores of Lake Idaho were made of granite. The lake lapped at the grainite and broke it down into sand. That sand, pressed in a soup of groundwater and minerals, became sandstone.

In some places around Boise, including Hulls Gulch and Military Reserve, the sandstone is so soft and crumbly officials post signs to warn people away for their own safety.

The sandstone from Table Rock is different, said Matson.

Geothermal water there broke down silica in volcanic rock to create an especially hard sandstone that's ideal for building - as Boise's century-old structures can attest.

Matson urges people to look closely at sandstone next time they're in the Foothills.

"There are a lot of details in the stories told by rocks - the fossils, little ripples, all the evidence of this being the margin of a lake. It helps one have a sense of place," he said.

Anna Webb: 377-6431

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