Glacial Geology of the
Last updated on June 04, 2008
In most general descriptions of glacial features, moraines are described as consisting dominantly of till a poorly sorted mixture of rock types. This till was supposedly deposited at the front of a glacier during a period when the front of the glacier was at a still-stand while melting (ablation) and advance of the glacier were in balance. However, in areas underlain by unconsolidated sediment, whether of glacial or pre-glacial origin, the moraines are commonly ice shoved moraines consisting of sheets of the underlying sediments thrust up into hills. Depending on the nature of the underlying sediments, these moraines may have little till.
Ice shoved moraines have the appearance of small scale mountains. This is because ice shoved moraines have also developed through the equivalent of thin skin tectonics.
As the glacier advances over underlying sediments it may push them in front in a manner similar to the action of a bulldozer. The glacier does not move as a single mass but as lobes or sub-lobes. Each sub-lobe develops a separate arcuate moraine in front of it.
Prof. Daniel Davis led the GEO 111 Environmental Geology class in an analog demonstration of glaciotectonics. The sublobes of the glacier are plexiglass or plywood sheets. The underlying sediments consist of flour. It was a typically humid Long Island day, so the flour is quite cohesive. Click on the thumbnail images for larger images. The zone of detachment along the north shore or to the north of Long Island could be the basement rocks in Long Island Sound, or possibly a permanently frozen clay rich layer which would be very competent, or just below a clay rich layer with a high hydrostatic head developed below it. Such a clay layer could be in the Magothy Formation or the Smithtown clay.
Factors influencing the development of Morphologically Prominent Glaciotectonic Features (from Aber and others (1989)
Elevated ground-water pressure within a confined aquifer
Ice advance over permafrost near ice margin
Ice advance against topographic obstacle
Lithologic boundaries in substratum
Surging of ice lobes
These are all factors that may have been important as the Wisconsinan glacier, which after advancing across the basement rocks of Connecticut and northern part of Long Island Sound, encountered the unconsolidated Coastal Plain sediments just to the north of present day Long Island.
Glaciotectonic features such as folds and faults in Cretaceous and Pleistocene sediments on Long Island were recognized in some of the earliest geological papers, although the authors may not have known how the features formed. Fuller (1914) describes many such features. Some thought the extreme deformation in the pre-glacial and glacial sediments may have been the result of orogenic (mountain building) forces (see discussion by Fuller, 1914, page 205). However, the fact that the underlying beds were not disturbed suggested that the forces could not be orogenic. Merrill, 1886, was one of the first to explain these features as resulting by glacier shoving. The title of his paper was "Some dynamic effects of the ice sheet".
Fig. 4 Sketches from Fuller (1914) showing deformation features in Pleistocene sediments. Fig. 3A is from Mather (1843) showing deformation in the Gardiners Clay and associated sediments. Fig. 3B shows deformation in the Gardiners Clay on Gardiners Island. Fig. 3C shows folded and faulted the gravels that overlie the Montauk till at Ditch Plains.
There have been continuing reports of glacial tectonic features. Several articles in the second half of the 20th century gave rather detailed descriptions of glaciotectonic features. For example, Mills and Wells, 1974, documented large-scale ice shove deformational features near Port Washington, NY. The deformed strata were exposed in large open pits in which there was almost a mile of exposure. Fig. shows a portion of the section. Mills and Wells (1974) interpretation of the history is that Montauk till and the Montauk outwash were deposited upon the Cretaceous sediments. The Montauk till was then overlain by the Roslyn outwash. After the deposition of the Roslyn outwash there was a glacial advance over the area during which Cretaceous sediments to the north were thrust as several large blocks over the pre-existing glacial deposits. The outwash was repeatedly faulted whereas the Montauk till formed large folds. Following the thrust faulting and folding the glacier advanced over the area depositing the Roslyn till. The Roslyn till is variable in thickness, in some places it is as much as 30 feet thick.. There is an angular unconformity underlying the Roslyn till in this area. The Roslyn till extends southward to the Ronkonkoma moraine. The lack of outwash deposits overlying the Roslyn till suggests a general stagnation of the glacier after it covered the area.
Sirkin, 1976, documented glaciotectonic structures on Block Island in which late Wisconsinan outwash (Qno) has been folded in the earlier Wisconsinan Montauk Till (Qm) which are then overlain by the late Wisconsinan till (Qnt) Fig. 5.
Oldale and O'Hara, 1984, documented large-scale ice shove deformational features in moraines on Cape Cod and the islands south and west of Cape Cod which were developed during the late Wisconsinan (about 20,000 years before present, b.p.). These moraines also occur on the Atlantic Coastal Plain which has a stratigraphy similar to that of Long Island. These moraines have also been correlated with those on Long Island (See Fig. above) Oldale and O'Hara conclude that the moraines in the vicinity of Cape Cod are not a result of sedimentation, but are a result of ice shoving. The recession of the ice sheet was viewed as stagnation-zone retreat followed by vigorous ice front advance. Upon advancement the underlying glacial and pre-glacial sediments were detached in blocks as much as one kilometer long, one-half kilometer wide and 30 meters thick. As the glacier advanced it over-rode the deformed sediments leaving a veneer of discontinuous till overlying the sediments (See Fig. below)
Fig. 7 From Oldale and O'Hara (1984) Fig. 2 A and 2B show the suggested sequence of events for one cycle of glacier stagnation and re-advance in the Cape Cod area. "a" is the ice. A downward arrow is for stagnant ice and horizontal arrow is for advancing ice. "b" is older outwash derived during the stagnation retreat stage of the glacier. "c" is younger outwash deposited during stagnation retreat. "d" is the till deposited discontinuously by the overriding glacier during advance. "e" is outwash younger then the moraine deposited during the following stagnation retreat phase of the glacier. Fig. 2C shows the stagnation retreat and advance positions of the glacier. Stagnation positions represent positions of outwash-plain building. Re-advance positions are the positions of moraine built by glacial tectonic processes.
Documented examples available on the web that show deformation associated with glaciotectonics on Long Island are in Ashley Schiff Park Preserve, Hither Hills, More Hither Hills, David Weld Sanctuary, and Ranco Quarry.