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Call it the Grotto of Black Hills Geology.

The historical rock grotto that sits beside a closed Catholic church in Oelrichs is actually a shrine to St. Therese of the Little Flower, but the structure is also an impressive monument to the geological history of southwestern South Dakota.

More than 15 different types of collectible minerals, gemstones, fossils and petrified wood are stacked and grouted into a structure that stands about 12 feet high, 16 feet long and 6 feet deep. The rocks that comprise it come from the prairie agate fields and Badlands fossil beds to the east and the Black Hills caves and mineral deposits to the west. It contains specimens of stalagtites and stalagmites, rose quartz, prairie agates, crystal geodes, schist, mica, selenite clusters, sandstone clusters, petrified moss, fossilized coral and much more. 

Rock hound Ellen Tilley, who also serves as president of the Western South Dakota Gem and Mineral Society, has never seen the Oelrichs grotto, but, like any rock hound worth her minerals, said she is anxious to visit what Oelrichs native and Sturgis resident Delores Muhm calls a "little gem hidden in Oelrichs."

"I've seen a lot of grottos, but not one more beautiful than this one," Muhm said.

The grotto also is unusual in that it honors the patron saint of missions and missionaries, not the more common Catholic icon, the Blessed Virgin Mary, in spite of what the historic marker in front of it says.

It was constructed between 1932 and 1934 by the Rev. Gerhard Stakemeier, then-resident priest at St. Martin Catholic Church and a rock hound himself, with the help of the only other bachelor in the parish at the time — who would become Muhm's father.

"My dad was the only one in the church that wasn't married at that time," recalled Muhm's brother, Nick Bogner, of their father, who homesteaded at Oelrichs in 1909. Using his wooden-spoked Buick to pull a Model T trailer, the elder Bogner made many trips to Wind Cave to load up rocks that were brought above ground as cave enthusiasts explored and enlarged Wind Cave.

"They brought a lot of those rocks out from the cave when they were widening rooms in the cave," Bogner said. "He must have made 20, 30, 40 trips up there for a load of rocks. It must have taken him a year and half."

Many of the other rocks came from the surrounding prairies, Badlands and ranches in the area, including an impressive selection of Fairburn agates — the official state gemstone of South Dakota. Over the years, vandals made off with those highly prized specimens, and today there are none to be found in it.

"There were quite a few Fairburn agates in it, but people chipped them out," Bogner said.

Bogner, 80, still lives on the ranch that his father homesteaded 103 years ago and he is the primary caretaker of the grotto, the church and the priest's house, all of which were placed or have been placed on state and national historical registers in 2002, after the church was closed as an active parish in 1999. 

One of the requirements of its historical designation is that St. Martin's grotto and church be open to the public at least 12 hours a year, so Bogner hosts an open house each Labor Day and Memorial Day. On Sept. 3, the buildings and grotto will be open from 9 a.m. to 3 p.m. to the general public, including rock hounds like Tilley.

Tilley say western South Dakota's unique geological history on display at the grotto is one of the reasons that it is ranked fifth in the nation for rock-hounding locations by national magazines in that field.

"South Dakota was part of a great inland sea at one time, which is why we have so many marine fossils," Tilley said.

The Black Hills, home to more than 140 minerals that can be mined, also make it a prime location to hunt rocks. That's something that King and Tilley have done all their lives. Gold, mica and feldspar are three of the most common minerals, and rose quartz is so prevalent — there is more of it in the Black Hills than any place in the world — that it has been named the official state mineral.

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"Why rocks? I guess I've been a rock hound all my life, or at least most of it," Tilley, 73, said.

She loves their beauty, their geologic history and the idea of getting outdoors to explore for them. "There's just such a sense of peace about it," she said.  

Rock collecting in the Badlands National Park is illegal today, but there's still plenty of public land in western South Dakota where it is not, Tilley said. She still explores many of the areas that Stakemeir likely did back in the 1920s and '30s, such as the Buffalo National Grasslands and places with names like Railroad Buttes and Indian Creek.

"There are acres and acres of rocks in places where you can still find prairie agates, Fairburn agates and ... beautiful little geodes with loose crystals inside called chalcedony that were formed from volcanic ash," she said.

Tilley is particularly fond of chalcedony, which are little geodes found in grasslands east of Scenic.

"They just weather out by the thousands and they're really rather ordinary looking unless you realize what they are," she said.

The rocks produce a tinkling bell-like rattle, thanks to the loose crystals inside that earned them the nickname Indian Rattle. Native Americans used the rocks to make percussion instruments for ceremonial purposes and, wrapped in hide, as baby toys.

"I've given away thousands of them," she said. "There's never any two alike."

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