Undersea Probe

"I feel," said Richard Montgomery Field last week, "like a hunter who has suddenly changed bow and arrow for a modern rifle."

The trim, white-hulled ketch Atlantis, research ship of the Woods Hole (Mass.)

Oceanographic Institution, was back in its snug port after a month at sea, bringing a rich haul of new knowledge about the submerged land off the Atlantic Coast -- harvest of an idea that the chunky, 50-year-old geology professor at Princeton University and his associates had been working on for three years. Dr. Field had no thought of learning anything new about the surface topography of the sea bottom. A great number of soundings with the old-fashioned line & sinker, more recently with the echo sounder, have disclosed that contour to oceanographers. Dr. Field wanted to know what lay beneath that bottom. I occurred to him to use the "artificial" earthquake method by which oil prospectors map subterranean rock structures. This involves setting off charges of dynamite, measuring the time required for the earth ripples to reach a seismograph planted some distance away, studying the wavy lines on the seismograph record (TIME. Nov. 4). This set-up is called a geophone. Transplanting it to the sea floor and making it work there was like doing a chemical experiment at the bottom of a swimming pool. Nevertheless the geologist got to work, enlisted the technical ingenuity of Dr. William Maurice Ewing of Lehigh University. It was necessary to waterproof the dynamite charges, steel-jacket the seismographs, rig electrical connections and a device for determining whether the recorders on the bottom were level. After making sure that everything was shipshape aboard the Atlantis, Dr. Field went ashore, left the survey in Dr. Ewing's hands.

First Dr. Ewing checked the profile of the well-known Continental Shelf, which slopes gradually eastward from the shoreline for 80 or more miles, ending in a cliff which drops sharply to the deeps. Next Dr. Ewing set about finding what the Shelf was made of. Every day, in good weather and bad, a small boat put off from the ketch, planted explosive containers on the bottom while the seismographs were dropped from the Atlantis. Dull booms rolled up from the abyss.

It was previously known, from dredged-up samples, that the surface of the Shelf was rock laid down by sedimentation in the Cretaceous era, 70,000,000 to 100,000,000 years ago. With no knowledge of how deep this layer was, it was thought that it thinned out eastward, exposing at or near the edge of the Shelf the basic granite foundation of the North American continent, some 1,000,000,000 years old. Dr. Ewing's twitchy seismograph needles now told him how thick the sedimentary layer was. Near the shore the thickness was 500 ft. But as he moved eastward, the layer, instead of dwindling as he expected, got thicker & thicker. At the brink of the Shelf the sedimentation was two miles deep. Thus it appeared that the Shelf was really not a part of the continental foundation at all, but simply a tilted ridge of petrified mud. The actual line of the basic rock beneath fell off directly from the coastline (see cut).

With this surprising information in hand, Dr. Field was able last week to paint a revised picture of what happened long ago:

The Shelf was first formed, as a gradual slope reaching far out into the deeps. Then, during the Ice Ages, the vast glaciers on land absorbed so much of Earth's water supply that the sea-level fell. At first, it fell quickly, as geological time goes, then more slowly. There was time for the surf to gnaw at the eastern face of the soft rock. As the coastline sank foot by foot, this gouging continued steeply downward, carving out the great submarine cliff that remains today.

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