division of late Precambrian rocks and time in North America (the Precambrian began about 3.8 billion years ago and ended 540 million years ago). Rocks of the Keweenawan System are about 10,700 metres (about 35,000 feet) thick, overlie rocks of the Huronian System, and underlie rocks of the Cambrian System; it has been suggested that the youngest Keweenawan rocks actually may be Cambrian in age. In the Lake Superior region, Keweenawan rocks consist of reddish sandstones, siltstones, shales, and some conglomerates. Great thicknesses of lava flows also occur; it has been estimated that about 100,000 cubic kilometres (24,000 cubic miles) of lava were produced. The burden of the great weight of lava caused the crust beneath to sag and produced the basin that Lake Superior now occupies. The Keweenawan System has been divided into Lower, Middle, and Upper series; the lavas are primarily concentrated in the Middle Keweenawan Series, whereas the Lower Keweenawan Series is dominated by sediments. The Keweenawan System is named for prominent exposures studied at Keweenaw Point, Michigan.