CONGLOMERATE

( Fr- conglomérat; Ger- Konglomerat; Nor- konglomerat; Rus- )

CONGLOMERATE

A. Conglomerate, "jasper puddingstone" clock (greatest dimension - ca. 25 cm) fashioned from a boulder of the Lorrain Conglomerate that was collected from reworked glacial drift off Richards Landing, St. Joseph Island (in northern Lake Huron), Ontario, Canada. H.W. Courtney collection. (© photo by H.W. Courtney)

DESCRIPTION: Conglomerate is a sack name for rocks that consist of rounded to subrounded fragments (pebbles, cobbles and boulders, which are usually referred to as clasts) surrounded by a finer grained matrix (e.g., sandstone) -- i.e., conglomerates are lithified gravels. (It seems prudent also to note here that some so-called conglomerates include as many or even more angular than round clasts and should be called breccia-conglomerates, conglomerate-breccias or even breccia.) In any case, both the clasts and matrices of many conglomerates consist of relatively resistant rocks and/or minerals such as quartzite and quartz.

The compositions, color(s), effective hardness, specific gravity, breakage and other properties of diverse conglomerates differ within a rather large range. Therefore, they are not listed here. This does seem to be a good place to note that so far as any use they have had or might have in the future, only conglomerates with pebble size clasts seem to fit that role. But, conglomerates with larger clasts have found use as, for example, ashlar and hewn building stone.

OTHER NAMES: Several conglomerates have been given formal stratigraphic names (see Appendix B, Glossary). Three examples are the Fincastle Conglomerate of central western Virginia, the Kanayut Conglomerate in the Brooks Range of Alaska and the well known Shinarump Conglomerate of Arizona. The following are examples of other names given conglomerates, most of which have been used as gemrocks:

Cherry cake conglomerate - a conglomerate, formally designated the Andre Point Grit, that crops out on, for example, Andre Point on the western end of Toco Bay, near the northeastern tip of Trinidad. The conglomerate -- a photograph of which is on the web site www.gstt.org -- is described as consisting of red ferruginous claystone pebbles (and a few white vein-quartz pebbles) within a "coarse, gritty, [off-white] quartzose matrix." (ibid.)
Kingstonite - name given to a conglomerate with its matrix and even some of its clasts, nearly all of which are pebble-size, replaced by native copper. This rock, which comprises parts of some of the ore bodies of the famous Copper District of Michigan's Keweenaw Peninsula, has been fashioned by lapidaries into such things as bookends (see Rosemeyer, 2001, Figure 32).
... puddingstone - one British and three American gemrocks frequently, though not formally designated puddingstones, follow:

      a) Hertfordshire puddingstone, named for its occurrences north in Hertfordshire (north of London) and nearby areas consists of flint pebbles within a light-brown (ginger-) colored quartz sandstone.
      b)Jasper puddingstone, formally named the Lorrain Conglomerate (see Figure A), crops out here and there in a belt that extends from west and north of Sault Ste Marie eastward beyond Bruce Mines, Ontario -- e.g., near the north end of St. Joseph Island (in Lake Huron), Ontario -- and occurs as pebbles, cobbles and boulders here and there in Pleistocene glacial deposits in Ontario, Michigan, Ohio, Indiana, Illinois and eastern Iowa.
      c) Roxbury puddingstone, also known as the Roxbury Conglomerate, from small quarries within Roxbury, which is now a part of Boston, Massachusetts is the official "rock or rock emblem" of the commonwealth of Massachusetts.
      d) Fairburn Conglomerate (name pending), which consists of red, yellow, tan, orange and lavender jasper pebbles surrounded by a quartzite matrix occurs near Fairburn, South Dakota (Mitchell, 1985).

Puddingstone jade, which has found some use as a gemrock, may or may not belong in this entry; I have not seen a specimen, a good photograph or a good description of this material; as noted in the NEPHRITE entry, it is said to consist of nodular-shaped masses of relatively light green nephrite within a darker green matrix, but what "nodular-shaped masses" and "matrix" mean is beyond my ken.
Tillite conglomerate (breccia, conglomerate-breccia or conglomeratic tillite, etc.) -- rocks formed by lithification of glacial tills. See Figure B.

B. Tillite boulder (width shown - ca. 38 cm), which is an example of the innumerable, diverse conglomerates and breccias that occcur in stream beds, glacial tills etc. here and there around the world. Attractive boulders such as this one have been used for such things as doorstops and cut and polished or otherwise fashioned to be used as parts for all sorts of decorative ornaments. This boulder, found in a glacial or glacio-fluvial deposit near Lewiston, Montmorency County, Michigan, was likely transported to Michigan from exposures of the Gowganda Formation located north of Lake Huron. The Gowganda Formation, once interpreted and widely referred to as the Gowganda Tillite, is now recognized to consist of several diverse rocks. Although the precursor sediments of these rocks are thought to have been deposited during and associated spatially with glaciation, only a few of the rocks represent glacial till. Consequently, none should be called a tillite unless proved to be such. The glaciation that occurred when the precurser sediments were deposited was approximately 1.8 billion years B.P. ; the glacial transport of the subsequently lithified and metamorphosed rocks of the Gowganda Formation into, for example, Michigan was during the last "Ice Age" within the last 75 thousand years B.P. (© photo by Kim Hoisington)

USES: Pendants, bookends, clocks and diverse ornaments; also as a facing stone -- e.g., glacially transported and deposited boulders of Lorrain Conglomerate are used in masonry, as landscape accents, and even as monuments (boulders, as found; mosaics made of small boulders and/or sawed and polished sections of those boulders; and also large boulders, as such or sawed and polished like medium sized tombstones). Also, though larger than the delimiting breadbox mentioned in the introduction, Welsh conglomerate fruitmills (see www.walcot.com).and conglomerate millstones from several localities -- e.g., the Cloyd Conglomerate from Brush Mountain, Giles County, Virginia -- seem noteworthy here; among other things, several of these "stones" have found use as, for example, landscape accents.

OCCURRENCES: Conglomerates occur within sequences of detrital sedimentary rocks, and metaconglomerates, many of which are virtually indistinguishable from sedimentary conglomerates, occur in the metamorphosed sedimentary sequences. And, both rocks are relatively common in unconsolidated deposits -- e.g., glacial debris and beach gravels -- derived from those strata; this is true because most conglomerates and metaconglomerates are relatively resistant to both chemical and physical weathering and erosion.

NOTEWORTHY LOCALITIES: See localities noted under OTHER NAMES subheading.

REMARKS: The designation conglomerate comes from the Latin con- glomerāre meaning to wind into a ball, apparently in allusion to these rocks' consisting of so-to-speak clustered masses of clasts and their surrounding matrix materials. This origin is intriguing in that I, and I suspect many petrologists, immediately thought of armored mud balls when we first saw this etymology given for conglomerate. Also, from the nomenclature standpoint, as already noted, the designation metaconglomerate is usually applied to metamorphosed conglomerates.

The term puddingstone, frequently applied to conglomerates in Great Britain, is usually applied only in vernacular elsewhere. To me, puddingstone is best given only certain conglomerate (if any). The conglomerates -- at least the ones I have seen -- that may warrant the name puddingstones differ from so-to-speak normal conglomerates in two ways: they have fewer pebbles per unit volume of the total rock, and the color(s) of their pebbles differ markedly from the color of the surrounding matrix. This usage would, it seems, be possibly useful and also in harmony with the origin of the term, which is said to have arisen because these rocks "look like plum pudding."

So far as gemrocks are concerned, the Lorrain Conglomerate, often referred to as Jasper puddingstone, is an especially good example of the just mentioned appearance. -- Most of its pebble-size fragments are red jasper whereas the matrix, which makes up the predominant part of the rock, is white or off-white quartzite. (It is of at least passing interest that this rock has been given several names by those who are familiar with it because of appearance and its rather widespread occurrence in glacial deposits in the upper midwestern United States. Of the several interesting ones I heard while plotting its distribution as boulders, cobbles and pebbles in glacial deposits (during the 1970s) the name that seemed to me to be the farthest out was "lipstick rock" -- apparently based on the red color of its jasper pebbles. (Perhaps a collection of these vernacular appellations should be recorded, eh?!)

Polymict (=polymictic) conglomerates -- i.e., conglomerates with pebbles (etc.) of several different rocks -- are of particular interest to the geologist as well as to lapidaries and others who fashion things from them. For geologists, congomerates frquently hold the many clues that may help them decipher the geological history of the region where they occur. For those who fashion things from them, many of these rocks have a strong positive aspect but also a noteworthy negative characteristic: On the positive side, many of these conglomerates are extremely attractive and each one -- indeed each piece -- is unique; these are great selling points for their products. On the negative side, each of the different rock types of the pebbles (etc.) may require somewhat different treatment so far as having an end product with surface(s) that appear equally polished (or whatever); but fortunately some polymict conglomerates do not pose such difficulties, and those that do provide the challenges that some lapidaries and carvers seem to thrive on.

I could not resist including the following from a web site to which my attention was directed. Entitled "Edible conglomerates," it is attributed to Luann Byerly as part of a Teacher's guide for a grades 4-9 science project in Iowa: "Conglomerates contain pebbles of different types of rocks in a variety of shapes and sizes. Popcorn balls can resemble real conglomerates by adding colored marshmallows, candy (like M&M's) and nuts. In addition, conglomerates are formed under pressure and cemented together by precipitated chemicals: similarly, popcorn balls are cemented together by the students' hands and a syrup mixture."(www.aea10.k12.ia.us/curr/science/sci4-9/RockMin/Rockm471) What an experience!!!

As already noted, the Roxbury puddingstone is the state rock of Massachusetts.

SIMULANT:

***Concrete with gravel aggregate - [An often suggested criterion is that matrices of concretes effervesce with dilute HCl whereas those of conglomerates do not. Although this is a fairly good criterion so far as distinguishing between most of these two materials, it is not absolute: Some conglomerates have matrices that effervesce with HCl. In my opinion, the best check, using only macroscopic examination, is to look for sporadic small holes, which represent entrapped air bubbles; they are relatively common around the pebbles of concretes whereas they occur only rarely, if ever, around pebbles of conglomerates.].

REFERENCES: No general reference. Dake, Fleecer & Wilson, 1938; Frondel, 1962

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