One cannot fully appreciate the immense size of the 25 foot overshoot metal wheel, unless you have the perspective of people beside it. When the wheel was put into operation back in the last century, it had to be shipped in, disassembled, via rail-car. Then it was reassembled on site at Turner Mill. This is a photo of the historic mill, as it looked around the turn of the nineteenth century: (I found it on the Internet)
The steel wheel is all that remains of Turner Mill, although parts of the rock shoot used to carry water to the wheel from the spring, is sometimes visible at the springs. The wheel was attached to a turbine that would supply the power needed to grind up corn and wheat. 
The Turner Mill Historic Site is near the Eleven Point National Scenic River. The Turner Mill Access (North) is located at river mile 21.5. Historical records indicate that when the mill was functioning at its zenith, families would bring their corn and wheat for processing, and then camp with their families along the Eleven Point River while they waited for their harvests to be ground up to take back home. ADDENDUM: I had the wonderful experience of kayaking the Eleven Point River in August 2020. Our group stopped at the Turner Mill access, and made the short hike up to the historic site. I took this photo of my "Paddling Partners" (Carol, Diane, and Ellen), as they stood high above the mill wheel.
The spring that flows out of the cave forms a brook that tumbles over rocks and ledges, to finally drain into the Eleven Point River. Addendum: When our group floated the Eleven Point River in August, 2020, the leader suggested we scout out this tributary stream that flows out of the cave, to see how far we could paddle up it. However, we could not get very far at the end that emptied into the river, as it was clogged with the growth of water-loving plants, that one would expect in the summer.
This photo shows some of the hikers navigating the rocky terrain along the stream.
The moss covered tree in this photo provided the perfect frame for the married couple I convinced to pose for me!
Turner Mill Historic Site sits on the edge of the Irish Wilderness, which is the largest of Missouri's eight designated wilderness areas. By having this designation, the land will be protected and preserved. The name comes from the fact that the land was settled by an Irish priest who, sadly, was NOT protected and preserved . Father John Hogan led a group of Irish immigrants to this area, desiring to escape oppression in St. Louis. However, during the Civil War, both Union and Confederate soldiers raided the Irish settlement, and Father Hogan, along with his group of settlers, were never heard of again. 
I am using this image of the spring gushing forth out of the rocks, as the visual aid to help me learn one of my First Place for Health memory verses from Isaiah 42:9 that says, "Behold, the former things have come to pass, and new things I declare; Before they SPRING forth, I tell you of them."
There is another great example of karst geology near Turner Mill, and it is called Grand Gulf State Park. The photo below illustrates how it is a "forked canyon", that is a remnant of an ancient collapsed dolomite cave system. This "grand gulf" stretches about 3/4 mile, between 130 foot high, rock walls. The area was declared a National Natural Landmark, in 1971, and is operated by the Missouri Department of Natural Resources. Some locals call it their "Little Grand Canyon".
Since being outdoors, hiking around these incredible points of interest in nature, can work up an appetite, it is comforting to know that there is a highly-rated eating establishment in Mammoth Springs, called RiverBend Restaurant. Our group ate here, and everyone was pleased with their meal. It sits right above the river, and those who so desire, can eat outside on their deck.
Mammoth Spring is one of the world's largest springs, with 9,7000,000 gallons of water flowing hourly, and it stays a constant 58 degrees Fahrenheit. The spring has been encompassed by Mammoth Springs State Park, since 1957. The Mammoth Spring is a National Natural Landmark, just as Grand Gulf is, in Missouri.
There is a trail that goes all the way around the ten acre lake, formed by the spring.
There is a restored 1886 St. Louis-San Francisco Train Depot as part of the Mammoth Springs State Park.
The train depot operates as a Railroad Museum now, and one of the park interpreters gave the Master Naturalists a very interesting program on the railroad history here.
The dam here was originally built in 1887 by the Mammoth Spring Improvement and Water Power Company. It was created to power a flour mill, cotton mill, and cotton gin. Starting in 1925, it was purchased by the AR-MO Power Company. They built a hydraulic facility to harness energy created by the dam, and operated it until 1972. In the photo below, the structure at the far end of the dam is the building that houses the Arkansas Visitor Center, rest rooms, and state park offices . One can use the walkway above the dams to cross over it, and take photos.
The heavy equipment was used to dig trenches for the underground irrigation system. That also was quite an upgrade from the watering system used by my grandparents---every family member carrying a bucket of water, and using a dipper to put a cup of water on each individual plant! It was quite a step up when my grandparents purchased a garden hose, with tiny holes in it, that would slowly wet the ground on a continuous basis---as long as someone turned on the water supply, that is. However, the system my son put in has a timer, so no human intervention is needed! Here is a photo of how the timer is set up:
