Kysten her øst for Hornbæk by ved Hornbæk plantage er interessant. Lidt inde i landet finder man den gamle stenalderkystlinie, den såkaldte Litorinakyst. Hvis man går over vejen og ind i skoven stiger terrænet brat, det er den gamle kystlinie. Engang var dette en stejl kystklint, men med tiden er havbunden hævet foran klinten og fremstår i dag som et såkaldt marint forland med standvoldssystemer. Strandvolde er opskyllet materiale, sand, ral og sten. Her ved Hornbæk er der primært tale om sand.
Når man går en tur ved de danske strande, ser man ofte mørke pletter eller striber i sandet. Mange mennesker tror fejlagtigt, at det er olie eller anden forurening, det er ikke korrekt men derimod en ganske naturlig forekomst. Det mørke sand er det, man populært kalder for tungsand. Da vejr og vind skifter, kan der være stor forskel på, hvor man finder tungsand.
Tungsand indeholder tungmineraler blandt andet: magnetit, ilmenit, granat, rutil, zirkon og titanium – og der kan endda findes guld i tungsand, men i meget små koncentrationer. Tungsand er nedbrudte sten fra det skandinaviske grundfjeld. Det mest almindelige sand i Danmark som er hvidt/gulligt er kvarts-sand, som er et let, men samtidigt et meget hårdt mineral. Det mørke tungsand er blødere og betydeligt tungere. Disse bløde og tungere mineraler males normalt mere fint end kvartssandet i havet. Magnetit er som navnet antyder magnetisk, og kan således udvindes ved at bruge en kraftig magnet til at udskille det fra sandet.
Man har forsøgt at udvinde tungsand flere steder i Danmark, men indtil nu har det ikke vist sig at være rentabelt. De forskellige mineraler kan udnyttes til forskellige ting lige fra briller (titanium) til slibemiddel (granat).
Her ved Hornbæk kan man lejlighedsvis finde tungsand på stranden. Tungsandet kan med sikkerhed findes i det åbne profil (det åbene profil er kanten fra plantagen ned til stranden) (se koordinater), hvor det er aflejret.
For at logge denne earthcache skal du løse et par opgaver. Bemærk du får brug for en kraftig magnet, et forstørrelsesglas og to beholdere eller poser til at samle sand i.
1. Besøg de to koordinater ved det åbne profil. Find det mørke tungsand i profilet og brug magneten til at samle lidt sand. Tag sandet af magneten og put det i den ene beholder. Gentag processen indtil du har ca. 2 teskefulde sand.
1A: Hvordan er tungsandet aflejret i profilet?
1B: Hvor er aflejringen tydeligst ved koordinat 1 eller koordinat 2?
2. Besøg koordinat tre og saml lidt af det hvide sand i den anden beholder.
2A: Sammenlign de to sandtyper, hvilken type sand har størst kornstørrelse?
2B: Hvorfor er der forskel på kornstørrelsen?
2C: Studér magnetit-sandet under lup. Hvilke farver ser du?
3. Forestil dig at du har et reagensglas og fylder vand og de to sandtyper i det. Når du ryster det kraftigt, hvilken type sand vil bundfælde først? Begrund dit svar.
Billeder af dig på stranden er meget velkommen. Venligst inden billeder af de åbne profiler.
UK:
The coast here east of Hornbaek town at Hornbæk Plantage is interesting. Here close to todays costline you can find the old Stone Age coastline, known as the Litorina coast. If you walk across the road and into the forest the terrain rises abruptly this it is the ancient coastline. Once this was a steep coastal cliff, but over time, the seabed is raised in front of the cliff and stands today as a so-called marine foreland with beach embarkment systems. Beach embarkments is material from the sea: cuttings, sand, pebbles and stones that has been washed up on the shore. Here at Hornbæk the beach embarkments are located mainly created of sand.
When you go for a walk by the Danish beaches, you often see dark spots or streaks in the sand. Many people mistakenly believe that it is oil or other contaminants, it is not correct but quite a natural occurrence. The dark sand is what is popularly called the heavy sand. As the weather changes, there may be significant differences in where to find heavy sand.
Heavy sand containing heavy minerals include: magnetite, ilmenite, garnet, rutile, zircon and titanium - and there may even be found gold in heavy sand, but only in very small concentrations. Heavy Sand is broken stones from the Scandinavian bedrock. The most common sand in Denmark is the white/yellowish sand that mainly contains quartz, which is a lightweight, but at the same time a very hard mineral. The dark heavy sand is softer and considerably heavier. These soft and heavier minerals is usually broken down more in grain size in the sea than quartz sand. Magnetite is as the name suggests magnetical, and can thus be recovered by using a strong magnet to separate it from the sand.
Attempts have been made to extract heavy sand in several places in Denmark, but until now it has not proven to be profitable. The different minerals can be utilized for various things ranging from glasses (titanium) and for grinding material (garnet).
Here at Hornbæk you can occasionally find heavy sand on the beach. Heavy sand can always be found in the open profile (the open profile is the edge of the marine foreland to the Sea) (see coordinates), where it is deposited.
To log this earth cache you must solve a few tasks. Note you will need a powerful magnet, magnifying glass and two containers or bags to collect sand in.
1 Visit the two coordinates of the open profile. Find the dark heavy sand in the profile and use the magnet to pick a little sand. Take the sand of the magnet and put it in one container. Repeat the process until you have approx. 2 teaspoons of sand.
1A: How is the heavy sand deposited in the profile?
1B: Where is the deposition most evident - at coordinate 1 or coordinate 2?
2 Visit coordinate 3 and collect a bit of the white sands in the other container.
2A: Compare the two sand types which of the sand types have the greatest grain size?
2B: Why is there a difference in grain size?
2C: Study the magnetite sand under the magnifying glass. What colors do you see?
3: Imagine that you have a test tube and fill water and the two types of sand in it. If you shaked it vigorously what type of sand will settle first? Justify your answer.
Photos of you at the beach is very welcome. Please no pictures of the open profiles.
Denne cache er frigivet i forbindelse med Earthcache 5k walk event.