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The building blocks of Kanaka Flats

People think of Jacksonville as a mining town. However the importance of building stones, used to assemble infrastructures in the historic old town, is often overlooked. They’re sort of the “poor children” of geology that pale beside their ostentatious brothers and sisters, gold and silver.

Building stone wasn’t transported far from its source, especially in the past when moving less valuable heavy rocks required more effort. A trip into historic Jacksonville reveals that many of the structures along the main street and, indeed, the historic courthouse, have foundations (and some basements) made of 90-million-year-old, greenish-brown sandstone of the nearby Hornbrook Formation.

The sandstone was relatively easy to quarry into flat rectangular blocks, which were transported a short distance from places like Kanaka Flats 1.3 miles south of the 4-way stop sign on Highway 238.

Hornbrook sandstone, and local granite, was quarried near Ashland and used in some of the older buildings. Because sandstone used as building blocks is more porous than granite, it tends to wick up water. That weakens the natural matrix between grains in the sandstone causing it to flake. Some store owners in Jacksonville have painted over the sandstone, but Mother Nature just guffaws and continues to flake the paint.

The superstructures of the buildings in historic Jacksonville are made of bricks. Those bricks came from local sources, one of which also existed at Kanaka Flats. The Flats was named for Hawaiians who lived there with American Indians and Portuguese. Kanaka in Hawaiian means “person” or “human being.” Placer mining by those people, excluded from Jacksonville by early settlers, was done at and near the site. The area was largely abandoned in the 1890s. Little remains of the original inhabitants' efforts except tailings piles and ditches that are now largely covered by houses.

Geologically, Kanaka Flats is a small, down-dropped fault block of Hornbrook sandstone and mudstone. One of the faults lies along the rather straight stretch of Jackson Creek (which is why the creek flows there). The sandstone is exposed along Highway 238 south of the road to Kanaka Flats. Just north of the road the rock looks different: it’s noticeably blockier and composed of greenish-gray, nearly vertical, 173 Ma, metamorphosed oceanic sediment. ("Ma" is geo-talk for “million years old.”)

The rock type is known as schist (be careful when pronouncing the word). Careful inspection of the sandstone will reveal broken, white shell fragments. If you stop to inspect the differences in the two rock types, please park in a wide spot off the road just south of the Kanaka Flats Road. Be careful on this heavily traveled road!

You can observe the sandstone building blocks either in Ashland (near the library, for example) or Jacksonville. It’s a nice diversion when you're in either town spending hard-earned cash or joining visiting relatives as tourists. By the way, speaking of diversions, 0.3 miles south of Kanaka Flats is Wagontrail Road. A whitish band is exposed in the steep road cut. That’s a rare occurrence of impure granular marble (like that exposed at Oregon Caves).

Thanks to Chelsea Rose of SOU who completed her master’s thesis on the Kanaka Flats archaeology and Nan Lindsley-Griffin who has graciously edited my text. All misteaks are my own.

Jad D'Allura is emeritus professor of the former Southern Oregon University Geology Department. If you have questions or comments contact him at rockit526@gmail.com.