Beyond Tysons Corner Shopping Center, past the rows of car dealerships and columns of high-rise office buildings that dot Leesburg Turnpike, there lies a link to the early 19th century.

Colvin Run Mill Park, an 18-acre historic site owned and operated by the Fairfax County Park Authority, has been steadily grinding and selling its own flour to customers from coast to coast since 1972.

Jeffrey Rainey, Colvin Run's resident miller and acting administrator, said one recent visitor bought 40 pounds of the freshly ground corn meal to store in his freezer in Georgia.

Even local restaurants have expressed interest in buying a regular supply of the mill's fresh corn meal and whole wheat flour, said Michael Rierson, superintendent of the Division of Historic Preservation, which oversees Colvin Run for the county park authority.

The three-story brick gristmill, located just off Rte. 7 on Colvin Run Road six miles west of Tysons Corner, produces 350 pounds of at least six different types of unprocessed flour (whole wheat, buckwheat, corn meal, rye flour, grits and bran) that supplies the mill's store for 10 days. The ground-up grains contain no preservatives, Rainey said, so they must be frozen to avoid spoilage.

"That's why we only make what we can sell in a week," he said.

Ironically, Rainey is allergic to the flour he makes and must wear a particle mask when he grinds to avoid severe asthma-like reactions to the dust and flour. "I have to stick my nose to the grindstone -- literally," he joked.

The 28-year-old Leesburg resident still considers himself a novice miller although he has practiced his unique craft for nearly 10 years. He also is learning how to construct the huge heavy wooden mechanisms that turn the grindstones, a skill, he says, "that will take the rest of my life."

Flour production is scheduled to start again this spring when it is warmer and there is more water in the nearby stream to power the mill's oak, redwood and cypress wheels.

When there is sufficient water and the mill's 3,400-pound grindstone gathers enough speed and rhythm, Rainey estimates he can churn-out about 450 pounds of flour in just a few hours. The flour is then bagged in specially designed pouches and stored in the park's freezer until it is sold at the on-site General Store.

In addition to the mill, for a nominal fee park visitors can explore a museum and an old-fashioned merchandise store that sells candy, 19th-century toys and kitchenware and 20th-century color film. There is also an empty barn used for the park authority's craft classes.

Store manager Michael McDonnell said he tries to make the store a learning experience for young visitors as well as a place that will turn a profit.

In the past six months, McDonnell said, he sold $560 worth of the mill's fresh flour, which comes in one-, two- and five-pound bags, andcosts from 62 cents (for one pound of corn meal) to $3.12 (for five pounds of whole wheat flour).

"We're a good deal ahead for the year," said McDonnell.

Revenue from the store's sales and other park events is channeled into a special trust fund administered by the county park authority. The money is then recycled back to the park at the beginning of each new fiscal year.

Rierson said Colvin Run's operating budget for the 1984-85 fiscal year is $90,000.

The park has already spent $12,000 this year to repair its gigantic 11-year-old main water wheel, which had deteriorated and weakened because of the nearby stream's high mineral content.

Fairfax County Park Authority bought the mill for $84,500 in 1965, and spent an additional $700,000 to restore the buildings to their early 19th-century appearance.

Rierson said the mill had been slated for demolition as the state highway authority planned to route Leesburg Turnpike through the area.

"Now, if you look at a map, the highway jags a bit to the right," said Rierson.

The mill is open weekends only, from 11 a.m. until 4 p.m. through March 15, after which it starts daily (except Tuesday) grinding operations, water level permitting. Admission is $2 for adults, $1 for children 15 and under and free for infants.

The park's preservationists emphasize there is more to the mill than turning flour into dollars.

"In every human being there is a need to understand and preserve the past," said Carol Deakin, volunteer coordinator for the Division of Historic Preservation. "We're an interpretive park now, not just a merchant mill, and so we show historical change as well as processes."