Frost on Beach Stones

There is plenty of time left for frost in the Keweenaw Peninsula. In Okemos, in south-central Michigan, the safe date is May 25 or so, but in the far northern reaches of Michigan, there will be plenty of frosty mornings to come well into June. Here's a shot of ice-cold beach stones delicately coated in frost near Hunters Point. The stones are mostly that ever present chert, the fine-grained sedimentary rock that is closely related to flint and jasper. Chert comes in several colors and is red on the beaches of the western U.P. because of traces of iron in the stone. I have read that chert was used for stone tools in prehistoric times, though I don't know how widespread its use was among the Native Americans who inhabited the shores of Lake Superior.