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Puddingstone, also known as either pudding stone or plum-pudding stone, is a popular name applied to a conglomerate that consists of distinctly rounded pebbles whose colors contrast sharply with the color of the finer-grained, often sandy, matrix or cement surrounding them. The rounded pebbles and the sharp contrast in color gives this type of conglomerate the appearance of a raisin or Christmas pudding. There are different types of puddingstone, with different composition, origin, and geographical distribution. Examples of different types of puddingstones include the Hertfordshire, Schunemunk, Roxbury, and St. Joseph Island (Drummond Island) puddingstones.
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Hertfordshire puddingstone
Hertfordshire puddingstone is a silica-cemented conglomerate composed of rounded flint pebbles and cobbles with matrix of fine sand and silica cement. The Hertfordshire Puddingstone is characterized by silica-cemented flint gravel that is brown to deep red in color and often exhibits black exteriors and thin rinds on cut or polished surfaces. It typically occurs scattered across the land surface as isolated concretion-like masses in the area of Hertfordshire and Plumstead Common, England. Large masses of Hertfordshire Puddingstone often occur within local Pleistocene glacial tills. More than a dozen large blocks of this puddingstone were recovered from Paleogene sediments during recent construction of the A10 bypass from Thundridge to Puckeridge. This indicates that the loose blocks of Hertfordshire Puddingstone were likely eroded out of these sediments. Although it is hypothesized that it is groundwater silcrete, its origin remains unresolved.
Schunemunk puddingstone
The Schunemunk puddingstone, which is exposed extensively on Bearfort Mountain, Boonton, Rockaway Township and Schunemunk Mountain, is a conglomerate that is part of a 3,000 feet (910 m) thick geologic formation formally known as the Skunnemunk Conglomerate. This puddingstone is a distinctive, Late Devonian, grayish-purple to grayish-red, thin to very thick-bedded, cross–bedded, conglomerate. Within the Skunnemunk Conglomerate, it is interbedded with grayish-purple to grayish-red sandstone, thin-bedded, medium-gray sandstone, and greenish-gray and grayish-red shale with mudcracks. This conglomerate consists of pebbles and cobbles of white vein quartz, red and green quartzite, sandstone, red and gray chert, and red shale. The grayish-purple to grayish-red conglomerate and sandstone is cemented largely by hematite and microcrystalline quartz. The cobbles that it contains range in size from 2.5 in (6.4 cm) to 6.5 in (17 cm). Pieces of Skunnemunk Conglomerate are easy to recognize and have been found in glacial deposits throughout the lower Hudson Valley region.
Roxbury puddingstone
The Roxbury puddingstone is a massive, Ediacaran, clast-supported pebble and cobble conglomerate that occurs within the Brookline Member of the Roxbury Conglomerate that is exposed around Boston, Massachusetts region. This conglomerate is composed of a grey feldspathic sand matrix and well-rounded pebbles and cobbles of quartzite, granite, felsite, and quartz monzonite. The beds of ‘puddingstone’ are complexly interbedded with layers of massive diamictite and laminated and graded argillite and sandstone. It likely accumulated as turbidites and submarine slumps within a submarine fan or outer slope environment within a deep rift basin submerged by marine waters.
Jasper conglomerate (St. Joseph Island and Drummond Island puddingstones)
Jasper conglomerate, which is also known as either pebble jasper conglomerate, St. Joseph Island puddingstone, Drummond Island puddingstone, or Michigan puddingstone, occurs on St. Joseph Island and the St. Mary's River area north and northwest of the Bruce Mines of Northern Ontario, about 65 kilometres (40 mi) east of Sault Ste. Marie. In these areas, it occurs as thick beds of Paleoproterozoic pebble conglomerate within the middle part of a geologic formation known as the Lorrain Formation of the Cobalt Group. The jasper conglomerate consists of subrounded pebbles of red jasper, black chert, white quartzite, hematite, and semi-transparent quartz in a coarse-grained quartzite matrix. It contains from about 30% to as much as 90% pebbles. Within the deposits of Lorrain Formation, the jasper conglomerates occur principally as the sedimentary fills of erosional troughs and channels of what are interpreted to be either alluvial fan or braided river deposits. These deposits are inferred to be nonglacial in origin and immediately postdate the Paleoproterozoic Makganyene glaciation.
Elsewhere, as on Drummond Island, Michigan, jasper conglomerate (Drummond Island puddingstone) occurs as loose gravel within Pleistocene glacial drift. These fragments of jasper conglomerate are glacial erratics that were eroded by continental ice sheets from Northern Ontario spread across Michigan, including Drummond Island, and as far south as Ohio and Kentucky during repeated glacial advances and retreats.