To my excitement I could collect not less than 10 (ten) different sands from only one single location!!! I couldn’t wait to take a quick snapshot and share it with you, although the sand must be now dried before filling the vials (some samples are still wet).

Perhaps shall I add here that all samples (about 10 ml each) have been collected from the loose material fallen on the ground without any disturbance of the original sandstone rock.

Here we joined one short geological tour around the university campus under the guidance of Prof. Heiner Siedel from the Institute of Geotechnology at TU Dresden. Prof. Siedel led us through several geological highlights and explained with great engagement and professionalism the origins and utilization of natural stones in the urban architecture of Dresden.

Sandstone (Posta type in the lower area and Cotta in the upper part) and shale stone

The first stop was right behind the Hörsaalzentrum where we could examine limestone rocks used for the pavement of small walking alleys. Next, Prof. Siedel explained the differences between porphyry and granite stones on Mommsenstraße and then we moved to the rectorate building. According to our guide, the entrance is flanked by a special “porphyry tuff” originating from Rochlitzer Berg, an unique geological formation in Saxony. Our interesting tour went further down the Helmholzstraße and Prof. Siedel showed us how the local sandstone was used in the construction of the university buildings. As sandstone is very sensitive to weathering I could collect a few ml of real sand accumulated at the base of the wall. The last stop was also very interesting: shale stone used for a historical monument, rather unusual if we consider the typical use of shale especially for the buildings’ roof.

All in one, our short geological tour was really great, we learned a lot about Saxon limestone, porphyry, granite, volcanic tuff and shale stones and we are very grateful to Prof. Siedel for organizing this tour.

House covered in shale rock (source: Wikipedia)

During out trip to Harz we met plenty of houses like this but also natural formations where we could observe the rock outcrops in their natural beauty. The Okerstausee lake is for example surrounded by such formations. On several places, the rock was weathered and transformed into small fragments of sand-like size so I just took a small sample for my Sand Atlas (sample no. 2295). The yellow color is actually clay brought by the tidal action onto the lake shores.

Shale rock fragments at Okerstausee (Lower Saxony, Germany)

This could be described on short as following (on the left – the approximate geological age in millions of years ago – mya-, on the right – the resulting rock type):

We walked along the hiking trail and stopped at several information boards. Each one focuses on a certain aspect of the geological transformation and you can observe some real hints just behind the panel.

Board no. 8 – Cretaceous epoch in Saxony

Our first stop was at a former quarry from the Cenoman period (part of the Cretaceous epoch – about 93 mya). The small quarry provided sandstone for the construction works in the surrounding area. Now vegetated, the quarry allowed me to take a small sand sample for the Sand Atlas: the orange colored sand reminded me of the sands from the Elbe Sandstone Mountains, fact confirmed by the information on the board – the sandstone belongs to the same plate as the formations south of Pirna about which I wrote in another post back in 2008.

Board no. 9 – Basalt volcanism in Tertiary epoch

The repetitive earthquakes created long cracks down to the earth mantle. The basaltic lava erupted and reached the surface in form of volcanic outbursts. Sometimes one can find several other minerals (including sandstone fragments) in the basaltic rocks such as pyroxene, olivine or feldspar. The basaltic fracture just behind the board no. 9 is very impressive:

Basalt quarry in Tharandt Open-Air Geological Museum (about 10 million years ago)

Board no. 7 – Spherical pitchstones

Like a game of nature, the several pitchstone boulders behind the information board no. 7 are very nice to look at and even more interesting to learn about. Pitchstone is a volcanic rock with dull, glassy aspect and very resistant to erosion. In comparison with obsidian (a volcanic rock with similar characteristics), pitchstone contains about 8% water in its structure.

Pitchstone in Tharandt Open-Air Geological Museum (about 310 million years ago)

The main color of pitchstone is black and, in case of Tharandt, the stone contains small red patches. Their presence in the main block is not yet clearly explained, some theories suggesting that they are foreign rock particles (most probably hematite – a red iron ore mineral) assimilated by the molten lava on its way to the surface. An important note: the pitchstones are protected by law in Tharandt Forest and cannot be collected. The photos above were taken without any disturbance of the stones’ natural position.

Board no. 2 – Phyllitic shale stones

Phyllite is a foliated metamorphic rock found in the surroundings of stones of the pre-Cambrian epoch. In Tharandt Open-Air Geological Museum we found phyllite behind the board no.2 in Mohorn-Grund (north of Tharandter Forest).

Phyllite in Tharandt Open-Air Geological Museum (about 370 million years ago)

Due to the high content in mica, phyllite has a nice silvery aspect but is very fragile and breaks easily just under the pressure of fingers (sample no. 2301 in Sand Atlas).

Magnified phyllite stone surface

Board no. 4 – Porphyry fan

Maybe the most spectacular geological formation is represented by the porphyry block at Mohorn-Grund in the northern part of Tharandt Geological Open-Air Museum. The area is just a small part of the volcano whose eruption 300 mya covered the whole Tharandt Forest area. The outburst had the form of glowing clouds involving gasses under enormous pressure.

Porphyry fan at Tharandt Open-Air Geological Museum (about 295 million years ago)

Porphyry formations in Tharandt Forest

The pyroclastic material sent into atmosphere by the dramatic explosion returned to the earth surface and melted by taking different shapes. The huge amount of material left behind an empty room inside the volcano body which filled with material from the surface. The rock fracture is impressive and attracts lots of tourists and visitors. Looking at the stone wall I notices an interesting area on the left side and climbed several meters to have a better look. One could see the single blocks of material similar to a basalt formation. From the top, the pluvial waters washed out the material and transformed it into a fine material, right perfect for a sample for the Sand Atlas (sample no. 2302):

2302 DE-SN

Unfortunately, we did not manage to visit the rest of information boards. We promised ourselves to do it next time since they offer even more interesting themes: ore mines (including silver ore), quartz deposits of Tertiary age, sands and gravels from Cretaceous, chalk pits and many more.

Note for the German readers of the Sand Atlas: a detailed geological map of the region and additional information about the geological park can be found here (in German only).

The nearby info tabs explained us that we find ourselves on a huge anhydrite deposit. Anhydrite (CaSO₄) is a white (sometimes grayish) mineral that, in contact with water, it transforms to the more common gypsum (CaSO₄·2H₂O). So the boulders we have seen were most probably made of old gypsum rock, assumption confirmed after arriving on the top of the hill. The soft gypsum rock was eroded by rain and wind action to a fine powder and I couldn’t pass by without taking a small sample for the Sand Atlas collection.

The image above shows the surface of a small conglomerate. The material was rather unconsolidated and turned into a beautiful white sand just by pressing with fingers. The dark tone is from lightning, the material is almost white in reality.

But, although surrounded by gypsum (photo left), Questenberg is known for another reason: the legend says that the castle lord lost his daughter in the nearby forest. The desperate father asked for help at the inhabitants of the surrounding villages and found the girl on the third day of Pentecost holiday with a crest with two tassels. The castle lord was extremely happy and payed a lot of favors to his people. Since that day, every year the inhabitants of Questenberg raise a wooden symbol on the gypsum mountain in respect to their former castle lord (figure right).

Small rounded seashell fossils from 285 million years ago in an old limestone quarry

The sample no 2204 US-HI from my Sand Atlas collection comes from Hana Bay on the far eastern side of the Maui island. Lava eroded from the volcano’s top was sprayed into the air. When meeting the strong winds coming from the ocean, the lava accumulated on the shores formed a small mountainous summit called also Ka’uiki Hill. The sand in the image above comes from there and, multiplied several times (width of the image is about 8 mm), it shows the amazing complexity of each and every sand grain.

The dream of every sand collector… star sand from Okinawa archipelago along the Japanese coastline. Thanks to Matt Whaley I also have a sample now from Taketomi island, absolutely beautiful. Nice… nice… nice…

The so-called ‘star sand’ is a biogenic sand made of skeletons of tiny, mono-celled sea creatures belonging to the foraminifera group (their scientific name is Baclogypsina sphaerulata). The star sand represents a touristic attraction not only among sand collectors but also ‘regular’ tourists search for it on the only two Japanese islands of Okinawa prefecture that contain it: Taketomi and Iriomote islands.

People searching for star sand on the Japanese island Iriomote (source: Wikipedia)