Читать книгу AMERICA - Joel Cook - Страница 71

The Delaware River above the "Forks," at the mouth of the Lehigh, breaks through a narrow notch in the Chestnut Hill ridge known as the "Little Water Gap," while farther to the northeast the ridge continues through New Jersey as the Jenny Jump Mountain. Above this is the noted "Foul Rift," where the river channel is filled with boulders and rocks of all sizes and shapes, the dread of the raftsmen who gave it the name, for many a raft has been wrecked there. But while this place is shunned by the navigator, it has an absorbing attraction for the geologist. This was where the great "Terminal Moraine" of the glacial epoch crossed the Delaware, recalling the "Ice Age," to which reference has already been made. When the vast Greenland ice-cap crept down so as to overspread northeastern America and northwestern Europe and filled the intervening Atlantic bed, it broke off many rocky fragments in its southward advance, scratching the surfaces of the ledges, and the fragments held in its grip, with striated lines and grooves in the direction of its movement. The ice steadily flowed southward, coming over mountain and valley alike in a continuous sheet, enveloping the ocean and adjacent continents, and finally halted on the Delaware about sixty miles north of Philadelphia. Its southern verge spread across America from Alaska to St. Louis, and thence to the Atlantic on the northern coast of New Jersey. Its southern boundary entered Western Pennsylvania near Beaver, passing northeast to the New York line; then turning southeast, it crossed the Lehigh about ten miles northwest of Mauch Chunk and the Delaware just below Belvidere. It crossed New Jersey to Staten Island, traversed the length of Long Island, and passed out to sea, appearing on Block Island, Cape Cod, St. George's Bank and Sable Island Shoal, south of Nova Scotia. The boundary of the glacier west of the "Foul Rift" on the Delaware appears as a range of low gravel hills, which are piled upon the slate hills of Northampton farther west, and reach the base of the Blue Ridge three miles east of the "Wind Gap." The boundary here mounted and crossed the Kittatinny ridge sixteen hundred feet high, being well shown upon its summit, and then passed over the intervening valley to the Broad Mountain or Pocono range. The Delaware at the "Foul Rift" is elevated two hundred and fifty feet above tide; and where the glacier boundary crossed the mountains in the interior it was at about twenty-six hundred feet elevation on the highest land in Potter County, the Continental watershed.

This vast glacier was so thick as to overtop even Mount Washington, for it dropped transported boulders on the summit of that highest peak in New England. Its southern edge in Pennsylvania was at least eight hundred feet thick in solid ice. A hundred miles back among the Catskills it was thirty-one hundred feet thick, and two hundred miles back in northern New England it was five thousand to six thousand feet thick, being still thicker farther northward. The Pocono Knob, near Stroudsburg, in Pike County, Pennsylvania, out-topped the glacier, and jutted out almost like an island surrounded by ice. The late Professor H. Carvill Lewis, who closely studied this glacier, has described how, all over the country which it covered, it dropped what is known as the "northern drift," or "till," or "hardpan," in scattered deposits of stones, clay, gravel and débris of all kinds, brought down from the northward as the ice moved along, and irregularly dumped upon the surface, thickly in some places and thinly in others, with many boulders, some of enormous size. It abraded all the rock surfaces crossed, and transported and rounded and striated the fragments torn off in its resistless passage. The line of farthest southern advance of the ice is shown by the "Terminal Moraine," stretching across country, which put the obstructions into the "Foul Rift." A glacier always pushes up at its foot a mound of material composed of fragments of rocks of all shapes and sizes, which the ice has taken up at various points along its flow and carried to its terminus, thus forming the moraine. This "Terminal Moraine" has been traced and carefully studied for four hundred miles across Pennsylvania, showing throughout a remarkable accumulation of drift materials and boulders, heaped into irregular hills and hollows over a strip of land nearly a mile wide. The action of the Delaware River currents at the "Foul Rift" has washed out the finer materials and cobblestones, leaving only the larger boulders and rocks to perplex the navigator.

Some of the performances of this great glacier in the region adjacent to the Delaware are remarkable. It has carried huge granite boulders from the far north and planted them all along the summit of the Kittatinny where it crossed. It has torn out big pieces of limestone, some of them thirty feet long, from their beds in Monroe County, north of this range, carried them in the ice more than a thousand feet up its steep northern face and over the summit, finally dropping them on the south side in the moraine in the slate valley of Northampton. These immense limestone rocks made comparatively short journeys, but one ponderous boulder of syenite from the Adirondacks was found in Northampton, well rounded and dressed, having travelled in the ice at least two hundred miles. There has also been found a "glacial groove" upon the rocks of the Kittatinny near the Water Gap, where some ponderous fragment, imbedded in the ice, as it moved along has gouged out a great scratch six feet wide and seventy feet in length. Although this ice had evidently resistless power in its slow motion, yet it seems to have had small influence upon the topography of the country. It appears to have merely "sand-papered" the surfaces of the rocks. It passed bodily across the sharp edges of the upright sandstone strata of the Kittatinny, yet has not had appreciable effect in cutting the ridge down, the glaciated portion east of the "Wind Gap" appearing as high and as sharply defined as the unglaciated part to the westward of the moraine. The glacier made many lakes north of the moraine, due to the "kettle holes" and obstruction of streams by unequal deposits of drift. It is inferred in the estimates of the duration of the glacier, from astronomical data, that the cold period began two hundred and eighty thousand years ago, the greatest cold being many thousand years later. The intense cold began moderating eighty thousand years ago, but the sea of ice remained long afterwards, and steadily diminished under the increasing heat. So many thousand years being required for melting, there are data inducing the belief that the ice-cap did not retreat from this part of the country back to Greenland until within ten thousand or fifteen thousand years ago. Then came the floods of water from the melting glacier, and it is significant that the Indians in the spacious valley northwest of the Kittatinny called that fertile region the "Minisink," meaning "the waters have gone," indicating their legendary memory of the floods following the melting and retreat of the glacier and the final outflow of its waters.