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.” [Illustration: _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.