The Pictured Rocks National Lakeshore’s Geology Field Notes page has this barely comprehensible stuff to say about Miner’s Castle:
The Miners Castle Member is a soft, crumbly, quartz sandstone (with abundant garnet content) about 140 feet thick; its complete section is exposed in the Pictured Rocks Cliffs between Sand Point and Miners Castle. Sediments of this member are generally poorly sorted.
Capping the easily eroded Miners Castle Member of the Munising Formation in the western half of Pictured Rocks, is the resistant Early Ordovician (480-500 million years old) Au Train formation. The Au Train Formation is a light brown to white dolomitic sandstone that forms the resistant cap to the underlying softer sandstones. The numerous falls in Pictured Rocks National Lakeshore are the result of this caprock.
Read on for much more about the geology of Pictured Rocks. Erosion is indeed a factor with one of the most visible instances being April 13, 2006, when one of the pillars of Miner’s Castle collapsed.