Stephen P. Broker
Cape Cod’s geology, ecology, and environmental issues are closely linked. The Cape was formed during the Pleistocene Epoch, beginning 21,000 to 16,000 years ago when the Wisconsin Glacier reached its maximum extent at what are now the south shores of Martha’s Vineyard and Nantucket Island. By 16,000 years Before Present, three adjacent lobes of the glacier had taken form, the Buzzards Bay, Cape Cod Bay, and South Channel glacial lobes. These stalled over the centers of Martha’s Vineyard and Nantucket, and in conveyer belt fashion they built up the land with glacially transported materials. A thousand years later, the lobes had retreated to positions centered on the Elizabeth Islands, the Sandwich moraine, and the Atlantic coastal plains of Truro, the Truro Highlands, Wellfleet, Eastham, and Nauset, and they built up these lands. With glacial retreat completed approximately 14,500 years ago, Cape Cod had assumed its basic glacial identity.
Present day geological map units for the Cape’s Pleistocene glacial deposits include the following: moraine deposits (sand and gravel, silt, clay, glacial till, and boulders - Sandwich, Buzzards Bay); pitted plain deposits (sand and gravel with some boulders - Mashpee, Wareham); outwash plain deposits (gravelly sand with some silt, clay, glacial till and boulders - Buzzards Bay, Harwich - occurring south of the moraines); ice-contact deposits (sand and gravel, till, and many scattered boulders - Dennis, Nantucket Sound - south of the moraine); coastal plain deposits (gravelly sand, silt, clay beds, and scattered boulders - Truro, Highlands, Wellfleet, Eastham, Nauset Heights, and Barnstable), and; lake deposits (gravelly sand with some silt, clay, and till - Cape Cod Bay). The coastal plain deposits of Wellfleet are significant to this unit, as they permit easy percolation of fresh water through gravelly sand, as well as easy percolation of contaminating toxic substances, pesticides, phosphate fertilizers, sewage, etc.
During the Holocene (10,000 BP to the present), the erosion of glacial deposits and the action of wind and waves gave further shape to Cape Cod. Geological map units of post-glacial deposits include: dune deposits (sand) on top of and along all cliffs of the Atlantic Ocean and Cape Cod Bay shore; beach deposits (sand, with some gravel and locally abundant boulders) formed by the action of wind and waves; sand and gravel deposits in kettleholes and valley floors; marsh and swamp deposits (decaying salt marsh plants mixed with sand, silt, and clay); artificial fill (human-deposited sand, gravel, and rip rap), and cranberry bogs (peat deposits topped by a layer of sand, often produced by the human conversion of white cedar swamps to agricultural land) (see Oldale and Barlow. 1986.) Beach deposits in Wellfleet include Jeremy Point, the north and south tips of Indian Neck, Mayo Beach, the northeastern tip of Lieutenant Island, and the northern tip of Wellfleet Bay Wildlife Sanctuary lands. Marsh and swamp deposits in Wellfleet include Duck Harbor marsh, the Herring River marshes, Great Island and Great Beach Hill Island marshes, Duck Creek, Indian Neck marsh, Lieutenant Island marsh, Blackfish Creek, and the Wellfleet Bay Wildlife Sanctuary marsh. Undifferentiated sand and gravel deposits of kettles and valleys include thin strips of land connecting Gull Pond (Wellfleet) and Slough Pond (Truro) to their adjacent marshes and swamps. Artificial fill in Wellfleet is limited to the town marina, although in earlier years there was a band of rip rap at The Gut, prior to its removal in the late 1970s by the National Seashore. A slide set described below focuses on the freshwater kettlehole, marsh and swamp deposits of Wellfleet and the artificial fill of the railroad dike over the mouth of Duck Creek.
Cape Cod consists of sand and gravel, silt, clay, boulders, and peat deposits. Higher elevations are occupied by forests, grasslands, and heathlands, and lower elevations by wetlands and swamp forests. Salt marshes are found along the Atlantic and Cape Cod Bay coastlines, protected by barrier beaches, embayments, and mouths of rivers. These change to brackish and freshwater marshes as one moves inland away from the tidal waters. A remarkably wide variety of freshwater wetlands is found at inland sites. Cape Cod has in excess of 350 freshwater ponds in all stages of development, from deep bodies of open water to quaking bogs and sphagnum bogs to wet meadows. A brief review of those upland habitats and freshwater wetlands occurring on the Outer Cape is useful for a study of Cape Cod’s critical environmental issues. I focus on the globally uncommon habitats identified in ecological studies of Cape Cod and mapped during the development of the Cape Cod Critical Habitats Atlas (VanLuven. 1990.)
The upland habitats of Cape Cod which are globally uncommon and globally significant are pitch pine/scrub oak barrens, critical woodland communities of hardwood trees uncommon or rare on Cape Cod, sandplain grasslands, heathlands, and grassy heaths. The Atlas defines sandplain grasslands as “grassy areas dominated by the tussock-forming bunch grasses little bluestem (Schizachyrum scoparia), and poverty grass (Danthoria spicata).” These are found only in the Upper Cape towns of Bourne, Sandwich, Barnstable, and Falmouth. Heathlands are “open areas dominated by dense, prostrate mats of bearberry (Arctostaphylos uva-ursi), black huckleberry (Gaylussacia baccata), and either beach heath (Hudsonia tomentosa) or golden heather (H. ericoides). These are found only in the towns of Eastham and Wellfleet, at the Coast Guard Beach area and the Marconi Beach/Wildlife Management areas, respectively. Grassy heaths are “assemblages of sandplain grassland and heathland species in which neither association is dominant.” These are restricted to Corn Hill and an area near Pond Village, both along the bayside of Truro. I have studied and photographed the heathland communities of the Marconi site in Wellfleet (see slide set #2).
The pitch pine/scrub oak barrens are “dry, open [canopy] pitch pine (Pinus rigida) forests which typically have almost impenetrable understories of scrub oak (Quercus ilicifolia).” Open areas are covered by bearberry, sedges, reindeer moss and British Soldier lichens. Tree-sized Black Oak (Q. velutina) and White Oak (Q. alba) are not found in these pine barrens, although they are widespread in the far more abundant and more densely vegetated pitch pine/scrub oak forests of the Cape. The pine barrens are found in Mashpee on the Upper Cape and on Great Island in Wellfleet, as well as in other Wellfleet and Truro locations. My studies of pitch pine barrens have centered on the Great Island area.
The critical woodland communities of Cape Cod are late-successional, broad-leaved or hardwood forests which persist only in remnant stands in two Upper Cape towns (Barnstable and Mashpee) one Mid-Cape town (Brewster), and the Outer Cape towns of Wellfleet and Provincetown. Broad-leaved tree species which are rare on Cape Cod are American Beech (Fagus grandifolia), Coastal Basswood (Tilia neglecta), American Holly (Ilex opaca), Yellow Birch (Betula alleghaniensis), Black Birch (B. lenta), Post Oak (Quercus stellata), and Hickory (Carya spp.). Outer Cape stands of critical woodland tree species are found in Wellfleet at the Wellfleet Bay Wildlife Sanctuary (a Post Oak/Hickory stand on Try Island), west of Herring Pond (American Beech), and in Provincetown where a large stand of American Beech is found on the edge of the Provincelands. The National Seashore has an easily accessible trail running through this Beech Forest. I have photographed all three Outer Cape sites. One must travel to Sandy Neck and Marstons Mills Airport in Barnstable and the Mashpee and Santuit Rivers in Mashpee to see well-developed stands of American Holly and Coastal Basswood. Yellow Birch and Black Birch stands were unmapped at the time of the Atlas Project, but Yellow Birch can be expected to grow in cool, moist areas (in association with red maple and Atlantic White Cedar), and Black Birch grows on moist slopes and along forest edges.
Freshwater wetland critical habitats include significant coastal plain pond shore communities, quaking bogs, sphagnum bogs, vernal pools and amphibian breeding habitats, Atlantic White Cedar swamps, and anadromous fish runs. I give brief descriptions of these habitats here. Lengthier descriptions of the other freshwater wetlands may be found in my 1995 Teachers’ Institute curriculum unit, Freshwater Wetlands of Connecticut. Coastal plain pond shores are found around many kettle ponds of the Cape, and they develop their greatest abundance of plant and animal life when the ponds dry down in late summer, exposing five to ten feet of wet sandy substrate that marks the transition from highest water line to lowest water line. The Atlas defines these as “critical habitat for many state-listed rare plant and wildlife communities” and “globally rare habitat.” Common and rare pond shore plants include grasses, sedges, rushes, and wildflowers (see representative species in slide set #2). The damselflies and dragonflies are among the most interesting and diverse of pond shore invertebrates, with nearly 100 species found on Cape Cod. Their names suggest their color and beauty, grace in flight, and predatory behavior: Ebony Jewelwing, Amber-winged Spreadwing, Violet Dancer, Bog Bluet, Civil Bluet, Barrens Bluet, Sphagnum Sprite, Blue Darner, Common Green Darner, Sand Dragon, Calico Pennant, Blue Pirate, Damson Skimmer, Goldenwings, Tenspot, Whitetail, Corporal Skimmer, Band-winged Meadowfly, Red Saddlebags. The smaller, more shallow Wellfleet ponds are particularly good for viewing damselflies and dragonflies: Turtle, Grass, Higgins, and Spectacle Ponds, and any associated vernal pools. Vertebrates of pond shores and surrounding uplands include Green Frog and Bullfrog, Painted Turtle, Snapping Turtle, and Eastern Box Turtle, and a number of birds, such as Great Blue Heron, Spotted Sandpiper, Least Sandpiper, Belted Kingfisher, Eastern Kingbird, swallow species, Gray Catbird, Cedar Waxwing, Pine Warbler, Common Yellowthroat, Red-winged Blackbird, Common Grackle, and American Goldfinch.
Quaking bogs are “floating mats of tangled roots and rhizomes which support thick accumulations of Sphagnum moss”, while sphagnum bogs are “perpetually saturated wetlands distinguished by unusually thick mats of Sphagnum moss and deep layers of peat.” Basically, quaking bogs have floating mats above open water, while sphagnum bogs are glacial kettles filled entirely with Sphagnum deposits. Truro has two classic quaking bogs, Featherbed Swamp and an unnamed bog just north of Round Pond in South Truro. I have taken core samples of the scattered, stunted pitch pine trees growing on the sphagnum beds and have determined that the trees, while only 6 to 15 feet high, have been growing for between 45 and 90 years at a rate of less than one inch trunk diameter per decade. Wellfleet’s only sphagnum bog is an extensive bog near Marconi Beach parking lot. This bog illustrates well the effects of nor’easters and hurricanes, as there are some sizeable pitch pine trees uprooted and fallen into the bog.
Vernal pools are temporary or ephemeral freshwater wetlands which typically fill up to a depth of 1-2 feet in February or March from snow melt and spring rains, exist as bodies of water for a few months, and dry up by late summer. They are breeding habitats for several amphibian species, including Marbled Salamander (Ambystoma opacum), Spotted Salamander (A. maculatum), Eastern Spadefoot (Scaphiopus holbrooki), and Wood Frog (Rana sylvatica), four species which will only breed in these fish-free habitats and thus are called obligate vernal pool species. A number of invertebrates are found in vernal pools, including the obligate species fairy shrimp (Order Anostraca). On June 14, 1998 during a Wilbur Cross High School field trip to Cape Cod, my students and I observed a breeding aggregation of spadefoots in a Wellfleet Village drainage ditch, a previously known site for this extremely hard to find species.
Atlantic White Cedar Swamps are critical habitats found in a narrow Atlantic coastal belt from southern Maine to North Carolina, with disjunct populations of white cedar (Chamaecyparis thyoides) to northern Florida and the Gulf Coast. On Cape Cod, they are found in Bourne, Barnstable, Yarmouth, Mashpee, Falmouth, Dennis, Harwich, Brewster, Orleans, Eastham, Wellfleet, and Provincetown. Atlas requirements for mapping cedar swamps included a minimum of 20 trees and a canopy cover of 50% or more. I have studied the cedar swamp at the Marconi site in Wellfleet, the type site for Cape cedar swamps. National Seashore literature indicates that this cedar swamp first developed 7000 years ago when a rising sea level intersected a dry kettle and created conditions necessary for the growth of sphagnum mosses and Atlantic White Cedar. Cedar swamps are dependent on periodic major disturbance for their long-term existence. Nor’easters and hurricane-induced blowdowns of trees, fires, or logging are required in order for these swamps to continue to exist. Without serious disturbance, white cedars eventually are outgrown and topped by hydric soil-loving red maples (Acer rubrum), which shade and kill the more light-dependent conifers. Severe competition between white cedars and red maples is very evident in the Marconi cedar swamp today, where red maple crowns fill every available gap in the canopy. The National Park Service underestimates of the age of the Marconi cedar trees at about 70-100 years old, a figure which is based on a limited number of trees cut from the swamp during the installation of a boardwalk or from tree cores. I have determined that some medium-sized white cedar trees are 130 years old and that others, twice as thick, are between 250 and 300 years old.
Cedar swamps are generally low in vertebrate species diversity, but one representative bird species of the Marconi cedar swamp is Whip-poor-will, a nightjar species which has been declining in its northeastern populations for the past several decades. Chuck-will’s-widow also occurs here annually, at least since the summer of 1996 (Broker, unpublished field notes), suggesting that breeding of this species on Cape Cod will occur and be documented before long.
Anadromous fish runs refer to small rivers and streams which connect freshwater bodies such as kettle ponds with estuarine and marine environments. Anadromous fishes are born in freshwater environments, migrate to the sea where they spend adulthood, and return to freshwater for breeding. The Cape’s three species of anadromous fishes are Blueback Herring (Alosa aestivalis), Alewife (A. pseudoharengus), and White Perch (Morone americana). Nearly all Cape towns have one or more anadromous fish runs. I have studied and photographed the Herring River in Wellfleet, which connects Herring Pond with Herring Cove and Wellfleet Harbor. The Herring River has received much ecological study and management attention in recent years, and efforts have been made recently to restore it to its former free-flowing condition.
