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A Cape Cod Notebook can be heard every Tuesday morning at 8:45am and afternoon at 5:45pm.It's commentary on the unique people, wildlife, and environment of our coastal region.A Cape Cod Notebook commentators include:Robert Finch, a nature writer living in Wellfleet who created, 'A Cape Cod Notebook.' It won the 2006 New England Edward R. Murrow Award for Best Radio Writing.

In the Wonders of our Landscape, Evidence of the Glacier


By now, the idea of the Cape’s glacial origins has pretty much fixed itself in the minds of most residents and visitors alike. But before Louis Agassiz put forward his revolutionary glacial theories in the mid-1800s, the idea of a continental ice sheet seemed as improbable to the general public as the continental drift theory did to their mid-20th century counterparts.

To our accustomed eyes and ears, however, the signs and evidence of the glacier that shaped so much of Cape Cod now seem everywhere and obvious. We look at the melt-water valleys along our south coast, or at the rocky ridges of the terminal moraine at Buzzards Bay and along Route 6a, or at such impressive house-sized boulders as Doane Rock in Eastham, and we say confidently, “Oh, the glacier did that.”

With similar ease we swallow and repeat the story of the formation of our kettle hole ponds. Oh sure, we say, these ponds are fossil icebergs. The depressions were formed when chunks of the retreating ice sheet broke off 12,000 years ago, forming these “kettle holes” which gradually filled up with water. Didn’t you know?

In fact, such strong believers in the glacial creation story have we become that we even accept the presence of glacial features we cannot see. A few miles north of Provincetown’s Race Point, for instance, and 80 feet beneath the waves, is Stellwagen Bank. Long known as a rich fishing and whale feeding ground, Stellwagen Bank is actually a 600 foot hill of glacial gravel – or so we’ve been told.

Now, as a child of my time, I don’t doubt these glacial theories. It’s just that sometimes we recite them with such conviction as though we ourselves could prove them. The truth is, on the face of it such geological stories are no less fantastic than the Wampanoag myths of giants creating our hills, valleys and ponds. And despite the scientific evidence for the glacial theories, most of us, if we are honest, take them on faith.

One of my favorite examples of the presence of the glacier here, though not as well-known as some other features, are the so-called “varved clay” deposits found in many places along the shoreline of Cape Cod Bay. These sedimentary outcroppings average 10 to 15 feet in height, and are composed of thin, wavery layers of blue clay interleaved with slightly thicker yellowish-red layers of sand, like some giant layered pastry. These layers, or varves, record the existence of the largest freshwater lake ever known on the Cape. Lake Shaler, as the prehistoric water body is known, occupied roughly the outline of Cape Cod Bay.  Today the bay is a three-sided embayment of salt water, but during the last ice age, its north side was enclosed by the retreating ice wall of the glacier itself, whose melting fresh waters poured into the basin on a seasonal basis.

In those Arctic summers, the run-off into Lake Shaler was copious and forceful, depositing large quantities of sandy material onto the lake bottom. In the winters, this runoff decreased, or stopped altogether, and the lighter silty material, mostly particles of blue clay, slowly precipitated down on top of the previous sandy runoff. This created the thin, alternating pairs of sand-clay layers that we find in the embankments lining the Bay shore, each pair recording the annual runoff from the glacier, much like tree growth rings. In time the ice sheet withdrew to the north and the ocean waters rushed in to fill Cape Cod Bay, leaving an exposed record of the glacier’s presence long its shoreline.

I like this story. More than that, I believe it. But - could I prove it to you?  

Robert Finch is a nature writer living in Wellfleet. 'A Cape Cod Notebook' won the 2006 New England Edward R. Murrow Award for Best Radio Writing.