Conglomerate – Nature’s Concrete
The coordinates bring you to a spot on a path in Ault Park.
Right in the middle of the path and just to the south side is an
example of conglomerate. There are more examples up the path at N39
08.348 W84 24.512. What we’d like to do here to compare and
contrast this natural formation to man-made concrete.
Concrete:
Just paces away at N39 08.343 W84 24.473 there’s a huge
example of man-made concrete. (Pictured below) Concrete is
considered a “composite” material in that on a
macroscopic scale it is obviously made from two different
materials. Compare this block to the blocks forming the foundation
of the rail trestle just beyond the block. The trestle blocks are
made from limestone. Limestone is formed by different crystal forms
of calcium carbonate, but to the naked eye they appear to be only
one homogenous material. But the concrete block obviously consists
of pebbles bound together by a matrix. The pebbles and sand are
called “aggregate” and the matrix is cement (probably
Portland cement). A major ingredient of cement comes from
limestone. Note the engineering of the aggregate in the block
– the large aggregate on the north side and the smaller,
finer on the south end where the rebar is.
Conglomerate:
The conglomerate rocks you see are very similar to concrete.
They too are composite material – here the
“aggregate” is referred to as “clasts” that
are held together by the much finer grain matrix. A material is
consider a conglomerate if its clasts are larger than sand –
greater than 2 mm in diameter.
Conglomerate material can be formed a number of ways: in oceans,
along rivers, or in this case a glacier may be responsible for the
assembly of clasts. The clast in this conglomerate was part of the
glacial “till” or the unsorted glacial sediment
container in the glacial ice. Unsorted sediment means that the
clasts will vary greatly in size. This conglomerate got its start a
quarter million years ago when a glacial melt water stream
deposited the stone and gravel clast at his location. Over the nest
several thousand years, water draining through nearby,
glacier-crushed limestone debris picked up calcium carbonate (lime)
and spread it as cement in this glacial outwash deposit. The
resulting cemented rock sediment, known as conglomerate, has slowly
been exposed as the surrounding streams have cut their valleys into
the local landscape. Note that limestone is the primary source for
conglomerate and concrete matrix!
Conglomerate classes: Two ways conglomerates are classified is
by texture and size of the clasts.
Texture:
Paraconglomerates consist of a matrix-supported rock that
contains at least 15% sand-sized or smaller grains (less than 2
mm), the rest being larger grains of varying sizes.
Orthoconglomerates consist of a clast-supported rock with less
than 15% matrix of sand and finer particles.
Conglomerates are also classified by the clast size of a
majority of the clasts:
• Granule conglomerate 2–4 mm
• Pebble conglomerate 4–64 mm
• Cobble conglomerate 64–256 mm
• Boulder conglomerate >256 mm
Logging requirements:
1) For the conglomerate at the posted coordinates identify its
texture from the description above.
2) For the conglomerate at the posted – how would you
classify it by clast size as described above?
3) Optional: If you like to commemorate your visit with a photo
I’d encourage one by the concrete block or the trestle
foundation stones versus the conglomerate – to discourage
internet earthcaching.