The Rocks

The Mineral Fork Tillite is a combination of fine-grained mud and silt surrounding larger pebbles, cobbles, and boulders that are both angular and sub-rounded. This poorly sorted debris was deposited into a shallow sea as a glacier bull-dozed, carried, and dumped rock debris while it advanced into the ocean. In addition, pieces of rock dropped into the mud on the ocean floor when floating blocks of ice-laden with rock fragments melted.  From this picture, we can see the larger rock fragments called dropstones suspended in the muddy matrix. Also, notice the wavy foliated texture that gives evidence of low-grade metamorphism that the rock underwent as they were buried by thousands of feet of overlying rock. (Photo by Rob Allen)