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Rock flour, or glacial flour, consists of fine-grained, silt-sized particles of rock, generated by mechanical grinding of bedrock by glacial erosion or by artificial grinding to a similar size. Because the material is very small, it becomes suspended in meltwater making the water appear cloudy, which is sometimes known as glacial milk .
Glacial striations are usually multiple, straight, and parallel, representing the movement of the glacier using rock fragments and sand grains, embedded in the base of the glacier, as cutting tools. Large amounts of coarse gravel and boulders carried along underneath the glacier provide the abrasive power to cut trough-like glacial grooves.
World War II pillbox on eroding boulder clay, Filey Bay, England Boulder clay cliffs in Gwynedd with Dinas Dinlle in the background. Boulder clay is an unsorted agglomeration of clastic sediment that is unstratified and structureless and contains gravel of various sizes, shapes, and compositions distributed at random in a fine-grained matrix.
Silt is detritus (fragments of weathered and eroded rock) with properties intermediate between sand and clay. A more precise definition of silt used by geologists is that it is detrital particles with sizes between 1/256 and 1/16 mm (about 4 to 63 microns). [2] This corresponds to particles between 8 and 4 phi units on the Krumbein phi scale.
Phosphorite, phosphate rock or rock phosphate is a non-detrital sedimentary rock that contains high amounts of phosphate minerals. The phosphate content of phosphorite (or grade of phosphate rock) varies greatly, from 4% [1] to 20% phosphorus pentoxide (P 2 O 5). Marketed phosphate rock is enriched ("beneficiated") to at least 28%, often more ...
Glacial till, in turn, is unstratified and unsorted debris ranging in size from silt-sized glacial flour to large boulders. [3] The individual rock fragments are typically sub-angular to rounded in shape. [4] Moraines may be found on the glacier's surface or deposited as piles or sheets of debris where the glacier has melted. [5]
Fault gouge is a type of fault rock best defined by its grain size. It is found as incohesive fault rock (rock which can be broken into its component granules at the present outcrop, only aided with fingers/pen-knife), with less than 30% clasts >2mm in diameter. [1] Fault gouge forms in near-surface fault zones with brittle deformation mechanisms.
Abrasion also produces rock flour which is visible in glacial outwash plains across New England. Maine has some of the longest eskers in the world. [12] As the climate began to warm, the glaciers began to melt and drainage from meltwater under the glacier formed huge torrents of sediment that, when compacted, left a long and sinuous ridge or kame.