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.
For many years geologists believed these valleys were carved out by glacial meltwater streams, but a more recent theory suggests a different origin, based on a phenomenon known as “spring sapping.” Spring sapping occurs when water emerges at the surface of a plain to form a spring, or seep, which then erodes back up the plain toward its head. There is evidence that during the final period of glacial retreat a large freshwater glacial lake existed to the east of Cape Cod. The surface of this lake may have been as much as 80 feet above present sea level, providing a powerful head of water that may have leaked through the porous outwash material of the Lower Cape, producing springs that created the valleys we see today. However they were formed, most of these valley beds are dry today, or else they carry only the small, sluggish streams that constitute Cape Cod’s “rivers.”
Kettle ponds formed over time as underground freshwater filled glacial depressions. Today the life in these ponds can be quickly affected by humans.
After the glacier’s final withdrawal, Cape Cod was only a rough sketch of its present streamlined form. Its moraines, high tablelands, valleys, and kame fields stood high and dry amid the surrounding coastal plain that stretched a hundred miles to the south and east, as far as the present-day Georges Banks.
The sea, however, continued to rise and advance, gradually filling the Gulf of Maine and covering the continental shelf. It probably reached the edge of the glacial Cape some 5,000-6,000 years ago. Now the rough-hewn shapes left by the ice began to undergo a process of planing, smoothing, elaboration, and elongation under the more subtle hands of the sea. Currents, tides, and storms began to chew away at the outer shores of the Lower Cape, creating in time what is surely the most impressive feature of the Seashore, and perhaps the quintessential Cape Cod landscape: the great marine scarp, or sea cliff, of the Outer Beach.
Stretching some 15 miles from Coast Guard Beach in Eastham to Head of the Meadow Beach in North Truro, this curved headland presents a nearly unbroken, undulating bulwark of bold, eroding bluffs, composed of clay and sand and ranging in height from 30 to more than 170 feet above the beach. From South Wellfleet to North Truro the smooth rim of the sea cliff is marked by a half-dozen pronounced dips or “hollows,” which are the eroded cross-sections of relict valleys. These hollows were later to play an important part in the human history of this beach.
To the casual observer the cliff face appears uniform, lying at a general “angle of repose” of some 27 degrees to the horizon. But look closely and you will see that, as the Colorado River has done through the Grand Canyon, the sea has cut open the geologic book of the Cape’s history, exposing thousands of years of history. Alternating layers of till—rock debris deposited by the ice—meltwater-deposited sand and gravel, and clayey silt tell of the glacial advance and withdrawal, of inundation and ebb by the sea. Large outcrops of blue clay, such as the 25-foot thick “Clay Pounds” just north of Highland Light, are the result of glacial lake sediments deposited during the last deglaciation.