During the second or Ordovician period of the Paleozoic era, the progressive submergence of Cambrian time continued until a climax was reached toward the middle of the period when fully four-fifths of the continent was submerged under shallow sea water. Since middle Ordovician marine strata are more widespread than the rocks of any succeeding age, we can be reasonably sure that so much of the continent was never again covered by the sea. In fact, so far as the records have been interpreted, this came nearest to being a universal flood in the whole known history of the continent. By the very character of the rocks deposited (seldom over a few thousand feet thick), we can be sure that the middle Ordovician continental sea was everywhere far shallower than the great ocean abysses of to-day. Because the lands were so low and restricted, relatively little land-derived sediment washed into the sea. But the shallow sea water was inhabited by millions of animals, the shells of many of which slowly accumulated to build up the thick bodies of limestone strata (Plate 14) which constitute the main bulk of rock of early and middle Ordovician age. The famous Trenton limestone, named from a locality in central New York, with its great abundance of fossils, was formed mostly by the accumulation of shells of animals during middle Ordovician time. Later in the Ordovician there was a considerable shift in level between land and water causing a withdrawal of much of the widespread sea. As a result of the generally more elevated lands, erosion proceeded more vigorously, and sands and muds were more abundantly deposited in the restricted sea, these sediments having consolidated to form the shales and sandstones which predominate among the upper Ordovician rocks.