| More than 350 mi('2) of patterned ground have been delimited in the Saginaw Lowland of east-central Michigan on the basis of the interpretation of aerial photography of varying scale, film type and date of acquisition. This phenomenon consists of an ordered network of broad, shallow troughs enclosing slightly higher centers. Individual forms range from circular features (79 feet in diameter) to elongated cells (1,289 x 480 feet). Most of the patterns, which can be classified as nonsorted nets, formed in somewhat poorly drained loam or slit loam drift on low-relief ((LESSTHEQ) 15 ft/mi('2)) surfaces which slope less than 3%.; The morphostratigraphic relationships of the Saginaw patterned ground to the Port Huron Moraine and proglacial Lake Warren I suggest that the nets formed not earlier than 13,000 yrs B.P., but are probably younger than 12,730 yrs B.P. The lower elevational limit of the nets is coincident with the shoreline of proglacial Lake Elkton (Lundy) in both the horizontal and isostatically uplifted terrain, indicating that active pattern formation was contemporaneous with, and bounded by, Lake Elkton. Early Lake Algonquin, the next proglacial lake in the sequence, came into existence about 12,400 yrs B.P. and provides a minimum date for the cessation of active pattern growth.; The Saginaw patterned ground is interpreted to be the result of thermal contraction-cracking and ice wedge development in periglacial permafrost. An ice-wedge cast, underlying the mesh of one of the patterns, provided supporting evidence for this conclusion. The pseudomorph is composed of a well-sorted, medium sand surrounded by a sandy clay loam to clay loam till. The abrupt boundary and marked textural difference between these two sediments suggest that the sand was deposited from above into a pre-existing wedge-shaped void in the till.; Ice-wedge fossilization was rare in the Saginaw Lowland, however. The somewhat poorly drained, fine-textured drift on which the patterns occur must have been conducive to the formation of ice-rich permafrost. Upon thaw, this material would have been prone to liquefaction, thereby severely restricting the opportunity for the replacement of wedge ice with allochthonous sediments.; The nonsorted nets in the study area provide geomorphic evidence of at least local discontinuous permafrost conditions during a brief interval (350-600 years) of the Lake Wisconsinan Substage. This morphogenetic conclusion implies a mean annual air temperature of -1(DEGREES) to -6(DEGREES)C in the Saginaw Lowland of Michigan during the waning Port Huron Stadial. |
