This is a view of an external erosion feature, one of many in Maquoketa Caves State Park. The park is made up of heavily eroded geologic features which were formed during and after the last Ice Age, about 12-14000 years ago.
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Such natural bridges are found throughout the United States wherever there is extensive erosion still present at the surface, including forests, deserts and even some oceanside bridges, such as the ones in Santa Cruz, California in Natural Bridges State Beach. The image shown in the link shows a collapsed bridge and only the remaining rocks.
This image likely dates from the 1940s.

This image shows a stone bench on the right, possibly used as a walkway, and on the left a dark rock outcrop that with some imagination can appear as a face. Above it there is a light colored formation which might also be construed as a face, though the characteristics are significantly more vague.
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The center of the picture appears to be a watercourse. This main cave passage is much more simple and underdeveloped than other such caves, without the obvious lighting, paths and handrails which were installed to more closely constrain visitor movement and increase safety.
This image likely dates from the 1940s.

This image follows on the previous one showing the same opening from the inside. The collapsed opening is also evident here, though it is too dark to tell if the water level is the same from this vantage point.
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Such natural formations as this were extensively developed as tourist attractions in places throughout the United States where they were found. Many became well known, either because of the stature as high quality cave formations, or through aggressive advertising on the highways and freeways being built.
This picture likely dates from the 1940s.

This image shows the watery and collapsed opening to the cave named Dance Hall Cave. This feature is part of the much larger Maquoketa Caves State Park in Maquoketa, IA.
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Caves such as these often were given imaginative names to encourage tourists to view them. It probably takes quite a bit of imagination to see a flooded cave such as this as a dance hall of any sort.
This image likely dates from the 1940s.

This image shows the road which passes the Iowa Training School for Boys, a reform school for what were then termed ‘juvenile delinquents’ located just west of Eldora, Iowa.
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Such ‘reform schools’ were established throughout the country during this period, with the growth of social services and the development of therapeutic programs for children who had criminal records.
This image is likely from the 1930s.

This image shows a newly constructed roadway bridge which appears to replace an older dirt and gravel route shown in the foreground.
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Such construction was a matter of pride and prestige in the 1930’s due to the constraints of the Great Depression in the United States. Such bridges as this allowed much more convenient, safe and speedy travel in rural areas which were still largely served with gravel roads.
This picture likely dates to the 1930s.

This image shows a gravel road as it crosses a water connection between Upper and Lower Pine Lake in Eldora, Iowa.
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The picture is not particularly scenic, but typical of postcard photographs of this era. Often photographers would attempt to document recent developments and construction projects in a park rather than the natural appearance as a way to entice tourists to visit.
This picture likely dates from the 1930s.

This image shows one of the dams that form the lakes making up Pine Lake State Park in Iowa. This is obviously Lower Pine Lake dam, as it has a road passing over it, and Upper Pine Lake dam has only a trail.
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The dam no longer has this appearance. Though built in 1922, it was modernized and the design significantly changed in 1991. However, the roadway remains in the same location. This image likely dates from the 1930s.

This image shows the lake nearby the previous posting, Upper Pine Lake, which is a short distance from Eldora, Iowa. It is upstream from Lower Pine Lake and also formed by a dam.
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This dam for this lake was constructed in 1934 and modified in 1990. This image likely dates from the 1930s.