Advancing, hesitating, and advancing again, the glacier moved as far south as Long Island, Block Island, Martha’s Vineyard, and Nantucket. Then, as the climate warmed, the front edges of the glacier began to melt; the ice sheet retreated north of the present outline of Cape Cod, dropping rocks and large chunks of ice as it went. Then, as cooler weather returned, it paused, and, as the flow of ice exceeded the rate of melting, the glacial lobes once more advanced. This time they pushed enormous amounts of drift material across the face of the Upper and Lower Cape, creating the Elizabeth Islands in Buzzards Bay and the rocky, morainal hills on the western and northern sides of the Upper Cape. [Illustration: _When the glacier melted, it left behind huge rocks, known as erratics, scattered on the landscape._] As the glacier began its final withdrawal about 18,000 years ago, water from the melting ice carried and sorted finer material, forming the gently sloping southern and interlobate outwash plains of the Upper and Lower Cape (see pages 26-27). Braided meltwater streams covered the ice chunks that the retreating glacier had dropped in thick, insulating layers of outwash material. Eventually, perhaps centuries later, the buried ice melted and the glacial debris above it sank, creating the hundreds of kettle holes that are characteristic of so much of the Cape’s topography. As the great ice sheets melted, the sea level began to rise, forcing the Cape’s fresh groundwater upward. When the freshwater level intersected with these kettle holes, the Cape’s numerous kettle ponds, so clear and deep, came into being. In some places these meltwater deposits and kettle holes form a highly irregular landscape of hills and hollows, such as the “kame and kettle” fields of Eastham, which can be seen on the bike trail from the Salt Pond Visitor Center to Coast Guard Beach. Farther north a prominent series of east-west valleys cross the high plains of Wellfleet and Truro, separated by the so-called “hogback” hills of Truro. Most of these valleys only partially cross the Lower Cape. An exception is the Pamet River Valley, which traverses the entire forearm of the Cape, separated from the ocean at its eastern end only by a fragile line of dunes that is sometimes broken through by major storms.