In the protected bays and inlets behind these elongating fingers of barrier beaches and islands, salt marshes—one of the Cape’s most characteristic and important ecosystems—began to form 4,000 to 6,000 years ago. Composed of a few species of salt-tolerant grasses, primarily the stalky cord grass (_Spartina alterniflora_) and the finer salt hay (_Spartina patens_), these green salt meadows of the sea built slowly on the accumulating sediment deposited in protected areas of water by the tides and land runoff. Gradually, as the sea rose relative to the land, the marshes raised and spread themselves on a platform of their own decay, forming thick beds of peat underneath them. As the climate continued to warm, the Cape’s woodlands evolved from boreal forests to the mixed pine-oak woodlands we see today. [Illustration: _Pitch pine is the most common tree on Cape Cod and the only species of native pine that grows in the National Seashore. It survives on well drained glacial sediments and stable sand dunes and is very fire resistant. In this stand on Great Island, the undergrowth is primarily bearberry._] As the Cape’s post-glacial environment grew more diverse and complex, so did the culture of the people living on it. The first Native Americans on Cape Cod are now thought to have arrived at least 9,000 years ago. By the late Archaic Period, starting about 5,000 years ago, local Indian groups had developed a seasonal pattern of movement based on multiple resources. During the warmer months they settled on the shores of bays, marshes, and fish runs, trapping birds and collecting herring and shellfish. In winter they retreated inland to the more protected forested shores of ponds and other wetlands. By the beginning of the Woodland Period, about 2,500 years ago, Indian settlements had grown even more numerous, larger, and more sedentary. About 800 years ago agriculture and a variety of new materials and tools had been introduced. One of the most important of these “tools” was fire; with it the Cape’s Native Americans changed the face of the land. Many early accounts attest to the extent and scale of the Cape’s original woodlands. Even at Provincetown, which must have always been the most barren area of the Outer Cape, the Pilgrim leader William Bradford observed “The whole countrie, full of woods and thickets, wooded with oaks, pines, sassafras, juniper, birch, holly, some ash, walnut, the wood for the most part open and without underwood, fit to go or ride in.” Archeologists now believe that the Cape Cod Indians may have practiced a form of low-level woodland management, burning underbrush, or wooded ridges, to provide browse for game and to make hunting easier. Such burns may have promoted the Cape’s characteristic pine barrens, tracts of open woodlands composed primarily of pitch pine, a tree adapted to periodic fires.