Geology topics

C. J. Patterson

Publications and source records attributed to C. J. Patterson.

4 recordsLinked to original sources

USGS2017

The significance of pre-existing, deeply weathered crystalline rock in interpreting the effects of glaciation in the Minnesota River valley, U.S.A.

Minnesota is largely underlain by Precambrian crystalline bedrock that was weathered to an average depth of 30 m prior to Late Cretaceous time. The fresh-rock-weathered-rock interface is irregular, with as much as 45 m of relief. Weathering exploited joints, locally isolating meter-sized volumes of rock known as corestones. Variable amounts of residuum were removed through glaciation to leave (1) saprolite overlain by an in-situ Late Cretaceous soil profile; (2) partially eroded saprolite; and (3) undulating fresh rock surfaces (commonly mantled by rounded boulders) that display striae and glacial or fluvial polish. Significant subglacial erosion of fresh bedrock is not required to form smoothly undulating bedrock surfaces with closed depressions; they may also form through removal of weathered bedrock and exposure of the weathering front. Large rounded boulders are not always shaped during transport; they may represent chemically rounded corestones resting at or near the bedrock source. Unambiguous evidence for glacial erosion includes striae and streamlining of bedrock parallel to striae. Polish on rock can be created fluvially, and smoothed grooves and ridges in the rock may be chemically produced. Many rounded boulders found in glacial till and strewn on bedrock surfaces probably originated as corestones.

Annals of Glaciology↗

USGS2017

Discharge of debris from ice at the margin of the Greenland ice sheet

Sediment production at a terrestrial section of the ice-sheet margin in West Greenland is dominated by debris released through the basal ice layer. The debris flux through the basal ice at the margin is estimated to be 12–45 m 3 m −1 a −1 . This is three orders of magnitude higher than that previously reported for East Antarctica, an order of magnitude higher than sites reported from in Norway, Iceland and Switzerland, but an order of magnitude lower than values previously reported from tidewater glaciers in Alaska and other high-rate environments such as surging glaciers. At our site, only negligible amounts of debris are released through englacial, supraglacial or subglacial sediment transfer. Glaciofluvial sediment production is highly localized, and long sections of the ice-sheet margin receive no sediment from glaciofluvial sources. These findings differ from those of studies at more temperate glacial settings where glaciofluvial routes are dominant and basal ice contributes only a minor percentage of the debris released at the margin. These data on debris flux through the terrestrial margin of an outlet glacier contribute to our limited knowledge of debris production from the Greenland ice sheet.

Journal of Glaciology↗

USGS1998

Laurentide glacial landscapes: The role of ice streams

Glacial landforms of the North American prairie can be divided into two suites that result from different styles of ice flow: (1) a lowland suite of level-to-streamlined till consistent with formation beneath ice streams, and (2) an upland and lobe-margin suite of thick, hummocky till and glacial thrust blocks consistent with formation at ice-stream and ice-lobe margins. Southern Laurentide ice lobes hypothetically functioned as outlets of ice streams. Broad branching lowlands bounded by escarpments mark the stable positions of the ice streams that fed the lobes. If the lobes and ice streams were similar to modern ice streams, their fast flow was facilitated by high subglacial water pressure. Favorable geology and topography in the midcontinent encouraged nonuniform ice flow and controlled the location of ice streams and outlet lobes.

Geology↗

USGS1997

Southern Laurentide ice lobes were created by ice streams: Des Moines Lobe in Minnesota, USA

Regional mapping in southern Minnesota has illuminated a suite of landforms developed by the Des Moines Lobe that delimit the position of the lobe at its maximum and at lesser readvances. The ice lobe repeatedly advanced, discharged its subglacial water, and subsequently stagnated. Recent glaciological research on Antarctic ice streams has led some glacial geologists to postulate that ice streams drained parts of the marine-based areas of the Laurentide Ice Sheet. I postulate that such ice streams may develop in land-based areas of an ice sheet as well, and that the Des Moines Lobe, 200 km wide and 900 km long, was an outlet glacier of an ice stream. It appears to have been able to advance beyond the Laurentide Ice Sheet as long as adequate water pressure was maintained. However, the outer part of the lobe stagnated because subglacial water that facilitated the flow was able to drain away through tunnel valleys. Stagnation of the lobe is not equivalent to stoppage of the ice stream, because ice repeatedly advanced into and onto the stagnant margins, stacking ice and debris. Similar landforms are also seen in other lobes of the upper midwestern United States.

Minnesota↗