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. [Illustration: _Low tide bares a wide expanse of sand patterns and tidepools at Coast Guard Beach._] The ocean also invaded parts of the land, creating estuaries and embayments at the mouths of these glacial valleys, such as those at Pamet Harbor and Blackfish Creek. Several of the Cape’s “salt ponds,” such as the one in Eastham below the visitor center, were originally freshwater kettle ponds that have been breached by the rising sea. As Cape Cod was being smoothed, slimmed, and invaded by the rising sea, it was also being lengthened. Like a sculptor working in clay, the ocean currents took much of the eroded cliff material and, carrying it both north and south, created the elaborate forms of the barrier beaches, barrier islands, and sand hooks that give the Outer Cape its characteristic filigreed coastline. Monomoy Island, North Beach, Nauset Beach, Coast Guard Beach, and Jeremy Point are some of the more prominent of these post-glacial landforms, enclosing such important estuaries as Pleasant Bay, Nauset Harbor, and Wellfleet Harbor. Among the more unusual formations are the tombolos of outer Wellfleet Harbor: the series of short sand beaches that connect Bound Brook, Griffin, Great Beach Hill, and Great Islands. Of all these sea-spawned parts of Cape Cod, however, the most impressive and extensive is undoubtedly the Provincetown Hook. These 3,000 acres of dunes at the northern tip of the Cape form a broad recurved spit of sand that encloses Provincetown Harbor, one of the finest deepwater harbors on the East Coast and the initial port for the _Mayflower_ Pilgrims in 1620. Some oceanographers believe that the hook began to form as much as 6,000 years ago and has built up in a series of roughly parallel ridges, or dune lines, widening out into the ocean.