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The Acasta Gneiss
The Acasta Gneiss is the oldest known intact fragment of the Earth’s ancient crust and represents one of
the few remnants of Archean North America.
The gneiss forms part of the Slave Craton, which is located in the north-western Canadian Shield.
Deposited during a long period of volcanic and metamorphic activity, the rock is classified as a tonalite
gneiss, which is defined as an intrusive igneous rock with a quartz content greater than 20%. Originally the
rock was a granitoid created 4.2 billion years ago in the Hadean era, just 300 million years after the
formation of Earth. Between 3.58 and 4.031 billion years ago (during the Archean) the granitoid was
metamorphosed into what is now the Acasta Gneiss.
The primary exposure of the Acasta Gneiss is on a small island approximately 200 miles north of
Yellowknife, a town located in the Northwest Territories of Canada where temperatures are often almost
-50 degrees celsius and is only accessible by sea-plane. As such the gneiss is expensive to collect but
worth it as no rock on earth is older. The specimens listed here were all obtained directly from the legal
owners of the mineral rights for the region.
|The Morton Gneiss is a Archean migmatitic gneiss that is the oldest rock in the United States.
It outcrops around Morton, Minnesota and has been dated to 3.6 billion years. Of a similar age
and appearance to the Acasta Gneiss, this another ancient rock that once formed the surface
of ancient North America. Typically has a pink hue like those shown below.
|Morton Gneiss specimens - £6/$10
Housed in a ~1.5 inch gem jar as shown.
Cambrian Beach Sandstone
|This is very interesting material that comes from the Mount Simon complex of Wisconsin and
represents a beach deposit from a series of barrier islands. The distinct banding in the
specimens is clear to see with a very fine grained layer alternating with a coarse sandstone,
which is indicative of a storm event. Considering this material dates to the early Cambrian it is a
wonderful record of island storms at a time when the explosion of life was well under way on the
continental shelves. Life was not yet present on the land and so the sandy beaches would have
been completely desolate, with the sand made up from eroded quartz rich rocks from even
further back in deep time. Nothing ever walked down these beaches and felt the sand between
their toes. Thought provoking stuff.
|Cambrian Beach Sandstone - £12/$20
Housed in a 2 x 2 inch glass fronted display case.
|Acasta Gneiss small
~3/4 inch - £10/$15
|Acasta Gneiss in display case
measuring ~6x4cm - £15/$25
|Mid Atlantic Ridge lava sample - £10/$15
Limited quantity remains.
Mid Atlantic Ridge
Ever since the destruction of Pompeii in the 1st century A.D. there has been
a fascination with this locality. Pliny the Younger wrote about the eruption a
few decades after the event as his uncle, Pliny the Elder, had died when the
volcano erupted. At the fall of the Roman empire the city was lost and
forgotten for over 1500 years. Now it is one of the most visited ancient
Roman towns in the world.
Mount Vesuvius is still believed to be active, but at present dormant, and
has rumbled on in the centuries since the destruction of Pompeii and
Herculaneum. These lava bomb pieces were all collected near to the crater
mouth and are typical of the volcanic rocks found across the entire
|Mount Santorini lava slice - £15/$25
The Mid Atlantic ridge is one of the most important geological features on the planet as it is a
zone of crustal formation on the ocean bottom where upwelling of magma from deep within the
earth is pushing the European and American crustal plates apart. The plates move at a rate of
only few cm a year, gradually widening the Atlantic Ocean.
The specimens listed here all come Ascension Island just 60 miles from the ridge. Barely one
million years old these are some of the youngest volcanic rocks on earth and were formed when
the islands were directly over the ridge, but have since been carried away by plate tectonics.
Mount Vesuvius lava slice - £10/$15
In antiquity Mount Santorini was known as Thera. It was an active volcano in
the second millenium B.C. with many people living within visible range of
the island. In around 1628 B.C. Thera erupted with one of the largest
explosions ever witnessed by humans. The eruption was of such magnitude
that it almost completely destroyed the island, leaving only a water filled,
fragmentary crater. The resulting devastation across the Mediterranean
resulted in the destruction of the Minoan civilisation - one of the first of the
ancient world. Their coastal towns and populations were decimated by the
tsunamis that followed the explosion and never recovered from the changes
to the landscape that occurred (fields flooded with sea water became to
salty for crops, the height of the land changed affecting the natural
harbours of the coast). Trade largely stopped and the few remaining
Minoans soon left the region to be absorbed into other cultures.
This Jurassic river delta sandstone from the Yorkshire coast of England.
Deposited by a large network freshwater rivers this sandstone is from an
era when dinosaurs walked in Yorkshire. Footprints made by dinosaurs
strolling across the river delta have been collected in the past,
inextricably linking the sandstone to the first emergence of dinosaurs.
Dinosaurs drank from, and waded through, the rivers that deposited
these sandstones, and the grains contained within the slices may have
even washed against a dinosaur's foot. Impossible to prove, but it is
images such as this that drive an interest in these rocks.
Each of these slices shows the banding formed by different flood activity
on the floodplain. Alternating bands of light and dark with varying
coarseness indicate periods of increased or decreased river flow as the
seasons changed. The bands can be counted to see the number of
(almost certainly) yearly events and the varying thickness of sediment
Varves are a remarkable form of deposition in that they often record daily, tidal or seasonal
variations in sedimentation. Typically deposited in lakes where there is less disturbance of the
sediments they are often linked with glacial lakes where the summer and winter periods are
recorded. In areas of higher sedimentation rates it is not uncommon for daily deposition to be
The grain textures of the varves show the differences between the seasons. Very fine grained
layers were deposited in winter when the only material available for deposition was what was
suspended in the water column. During the warmer seasons when water bearing sediment
was flowing into the lakes coarser grained bands were deposited.
The specimens listed below are from over one billion years ago and record between 20 and 40
years of sedimentation in the Belt Supergroup of Canada. Each specimen comes in a display
case as shown.
showing 20 years of
showing 40 years of
Devonian Varves - £5/$8 each
From the Devonian of the United Kingdom, these
varves were deposited 400 million years ago in
the large river system that covered much of the
land at the time. From the same period as when
the Old Red Sandstone of Scotland was laid
Jurassic Floodplain Sandstone
Mesoproterozoic Varves Thin Section £10/$15
These specimens are thin sections of Belt
Supergroup varves. Each has been ground and
polished so that it is thin enough for polarised
light studies. The nature of the layering is well
The Lewisian Gneiss is the oldest rock in the United Kingdom, dating back 3 billion years.
Exposed on the Isle of Lewis (from which it gets its name) and other western isles, the rocks
were laid down during the Archaean. The Gneiss forms part of a metamorphic complex that
is included in the North Atlantic Craton (the previously described Acasta Gneiss is part of an
older Slave Craton). The 'youngest' rocks in the Lewisian complex date to the early
Palaeoproterozoic (1.7 billion years ago) and represent the basement rock on which the later
Torridonian sedimentary event was deposited (below).
|Lewisian Gneiss slice in display case
|Lewisian Gneiss thin section suitable for
polarised light studies - £10/$15
The Torridonian Sandstone was deposited approximately 1.2 billion
years ago and represents the oldest sedimentary rocks in the
United Kingdom. The rocks are mainly red and brown sandstones,
some of which are made up of eroded Lewisian Gneiss, upon which
it is deposited.
The uneven (mountainous) surface of the Lewisian Gneiss was
gradually filled by the Torridonian Sandstone through erosion and
deposition during a period of prolonged fluvial activity. This resulted
in the sandstone complex varying greatly in depth as it filled the
valleys and depressions of the Lewisian Gneiss. When deposition
ceased, the Lewisian Gneiss was completely covered and the
landscape had become a flat, alluvial plain with most evidence of
the earlier hills and mountains being obscured.
(Stoer Group) slice in
display case - £6/$10