NA KOORDINATAH ZAKLADA
Na Glavnem trgu so med prenovo med drugim našli tudi tri kamnite krogle, izdelane za katapult. Krogle imajo premer 20 cm. Ali so bile samo pripravljene za obrambo srednjeveškega mesta ali morda uporabljene proti branilcem Kamnika, nam ni znano. Po prenovi trga so ob vhodu na dvorišče hiše z naslovom Glavni trg 1 pustili kar znatno večjo kamnit kroglo. Ali bi tudi ta lahko bila namenjena za obleganje srednjeveškega mesta s katapultom ali njegovim naprednejšim tehnološkim bratrancem tribokom*?
* Tribok je manj znan slovenski izraz za katapultu podobno, a tehnološko naprednejšo izmetno napravo, ki je bolj poznana pod francoskim (in tudi angleškim) izrazom trebuchet.
Na koordinatah tega geološkega zaklada lahko vidite precej veliko skalo lepo zaobljenih oblik, v obliki skoraj prave, nekoliko sploščene krogle. Podrobnejši ogled pokaže, da gre za kamen iz apnenca. Glede na velikost kamna je zelo malo verjetno, da so ga pripeljali od daleč. Povsem možno je, da je bil na prostor današnjega Glavnega trga odložen v enem od ledenodobnih obdobij.
OBLIKOVANJE KAMNITE »KROGLE«
Kdo je apnenčevo skalo oblikoval tako lepo na okroglo? Očitno ne gre za delo človeških rok ali orodja, zato je boljše vprašanje, kako so jo oblikovali naravni procesi.
Velik del Kamniško Savinjskih Alp sestavlja apnenec, zato za izvor kamnite krogle z veliko gotovostjo lahko določimo pobočja ob zgornjem porečju Kamniške Bistrice. Možna oblikovalca obrušenega prodnika v obliki kamnite krogle sta dve: ledeniško delovanje ali vodno delovanje. Ali kombinacija obeh procesov.
Ledeniško oblikovanje?
V kolikor se je oblikovanje krogle pričelo v ledeni dobi, je lahko ledenik odtrgal kos kamnine s pobočja ali dna doline po kateri je drsel. Naslednja možnost je, da je spodjedel kamnito pobočje in povzročil porušitev kamnine. Tako ali drugače je naš kos kamnine pristal na ledeniku in prej ali slej pod njim. Med premikanjem ledenika je kamen drsel po kamniti podlagi, se brusil ob ostalih kamnitih kosih in na ta način postopoma izgubljal svoje ostre robove, postajal vse bolj zaobljen. Ledenik bi ga lahko odložil v robni ali čelni ledeniški moreni ali pa je obležal v dolini kot ostanek materiala po stalitvi ledu.
Oblikovanje s pomočjo delovanja vode?
Večji kos kamnine se je v nekem obdobju odlomil od pobočja in se takoj ali pa s pomočjo kasnejših procesov znašel v strugi vodotoka. Gorski vodotoki imajo zaradi velikega vzdolžnega padca veliko energije in lahko prenašajo tudi večje kose kamnine – če ne ob običajnem vodostaju pa vsaj ob povečanem, spomladi ob taljenju snega ali po večjih padavinah. Kamen se je v tem primeru obrusil med transportom po rečni strugi – med drsenjem in zaletavanjem v kamnito dno ter ob trenju z drugimi kamni, ki jih je prestavljal vodni tok. Nižje v dolini, kjer je zaradi manjšega padca vodni tok šibkejši, je voda najprej odložila kamne z največjo maso.
In naša kamnita krogla?
Če predvidevamo, da kamnite krogle v bližino sedanje lokacije ni pritovoril človek, je najverjetnejši dejavnik oblikovanja vodni tok sam ali v kombinaciji z delovanjem ledenika.
Ali je ledenik po dolini Kamniške Bistrice kdaj segal vse do Kamnika, ni povsem gotovo. Površinski sledovi jasno kažejo, da je ledenik segal vsaj do sotočja hudournika Korošica s Kamniško Bistrico (v bližini spodnje postaje gondole za Veliko planino). Naplavinske terase in poplavne ravnine, ki so nastale med posameznimi obdobji ledene dobe, pa segajo tudi precej više od mesta sedanjega Glavnega trga. A novi model, ki so ga strokovnjaki izdelali na podlagi geoloških, geomorfoloških in ostalih podatkov kaže na možnost, da je v času največje poledenitve ledeni pokrov segal celo do okolice današnje Ljubljane. V tem primeru bi torej tudi naša krogla lahko na današnje mesto pripotovala z ledenikom.
JE BILO MOŽNO TAKO KROGLO UPORABITI ZA KATAPULT ALI TRIBOK?
Krogle za katapult, ki so jih našli med obnovo trga, imajo maso približno 10 kg. Po več virih sodeč je bila povprečna masa krogel uporabljena na srednjeveških katapultih med 10 in 20 kg.
Najzmogljivejši katapulti, oziroma njihovi tehnološki bratranci s protiutežjo, triboki, so zmogli zalučati predmete z mnogo večjo maso. Podatki niso povsem enotni, a ocenjujejo, da je bila največja zalučana masa tudi med 900 in 1300 kg (ni znano na kakšno razdaljo). Novejše rekonstrukcije srednjeveških tribokov s protiutežjo so zmogle na razdaljo 80 m izstreliti največ 476 kg.
VPRAŠANJA ZA VPIS ZAKLADA
- Kdaj najkasneje se je pričelo oblikovanje kamna v kamnito kroglo, če je pri tem sodeloval ledenik? Za odgovor bo treba seči v znanje iz geografije v osnovni šoli. Ali pobrskati po spletu. Pravilen je eden od naslednjih odgovorov: pred 10-15 tisoč leti, 100-150 tisoč leti, 10-15 milijoni let, 100-150 milijoni let.
- Kolikšna sta premer in masa kamnite krogle? Izmerite ali ocenite premer. Na podlagi meritev izračunajte približno maso. V izračunu upoštevajte, da ima 1 kubični decimeter apnenca maso približno 2,6 kg (1 m3 pa torej 2600 kg). Pri izračunu si pomagajte z matematično formulo za prostornino krogle: V = 4/3 π r⊃3;, kjer je V = prostornina in r = polmer.
- Ali bi to kamnito kroglo lahko na razdaljo 80 m izstrelili z največjo moderno rekonstrukcijo srednjeveškega triboka?
- Kamnita krogla ima novodobni dodatek, ki ga je prispeval človek. Bolj natančno poglejte površino in sporočite kaj je to.
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ZAKAJ ANGLEŠKI IZRAZ »THREE MAN BOULDER«?
Izhaja iz ocene, da so za premikanje takega kamna potrebni trije odrasli moški. Izraz uporabljajo ameriški prodajalci materialov za krajinsko oblikovanje. Velikosti kamnov označujejo tudi s »Half Man«, »One Man«, »Two Man« ... do velikosti »Six Man«.
ENGLISH
AT THE COORDINATES OF THE EARTHCACHE
During the renovation of Glavni trg, three stone balls made for catapults were also found. The balls have a diameter of 20 cm. Whether they were only prepared for the defense of the medieval city or perhaps used against the defenders of Kamnik, we do not know. After the renovation of the square, a significantly larger stone sphere or stone ball, was left at the entrance to the courtyard of the house Glavni trg 1. Could this also have been intended for the siege of the medieval city with a catapult or its more advanced technological cousin, the trebuchet?
At the coordinates of this EarthCache, you can see a rather large rock with rounded, almost spherical shape. A closer look shows that it is a limestone rock. Given the size of the rock, it is very unlikely that it was brought from afar. It is entirely possible that it was deposited on the site of today's Main Square during one of the ice age periods.
FORMING OF THE STONE "BALL"
Who shaped the limestone rock so beautifully into a round shape? It is obviously not the work of human hands or tools, so the better question is how it was shaped by natural processes. A large part of the Kamnik-Savinja Alps is made up of limestone, so we can determine with great certainty that the slopes along the upper Kamniška Bistrica River basin are the origin of the stone ball. There are two possible designers of the rounded pebble in the form of a stone sphere: glacial action or water action. Or a combination of both processes.
Glacial formation?
If the formation of the ball began during the Ice Age, the glacier could have torn off a piece of rock from the slope or the bottom of the valley along which it slid. Another possibility is that it eroded the rocky slope and caused the rock to collapse. One way or another, our piece of rock landed on the glacier and sooner or later under it. As the glacier moved, the rock slid along the bedrock, grinding against other rock pieces, and in this way gradually lost its sharp edges, becoming more and more rounded. The glacier could have deposited it in a marginal or frontal glacial moraine, or it could have been deposited in the valley as a remnant of material after the ice melted.
Formed by the action of water?
A larger piece of rock broke off from the slope at some point and immediately or through later processes found itself in the bed of the watercourse. Due to their large longitudinal drop, mountain watercourses have a lot of energy and can also carry larger pieces of rock – if not at normal water levels, then at least at increased levels, such as in spring when snow melts or after heavy rainfall. In this case, the rock was worn away during transport along the riverbed – while sliding and crashing into the rock bed and by friction with other rocks that were moved by the watercourse. Lower in the valley, where the water flow is weaker due to the smaller drop, the water deposited the stones with the greatest mass first.
And our stone sphere?
Assuming that the stone sphere was not brought to its current location by humans, the most likely formation factor is the water flow alone or in combination with the action of a glacier. Whether the glacier along the Kamniška Bistrica valley ever reached all the way to Kamnik is not entirely certain. Surface traces clearly show that the glacier reached at least as far as the confluence of the Korošica torrent with the Kamniška Bistrica (near the lower station of the Velika planina gondola). The alluvial terraces and floodplains that formed between individual periods of the ice age also extend much higher than the site of the current Glavni trg. However, a new model, which experts have created based on geological, geomorphological and other data, suggests that during the greatest glaciation, the ice cover even reached the area around present-day Ljubljana. In this case, our sphere could also have traveled to its current location with the glacier.
WAS IT POSSIBLE TO USE SUCH A BALL FOR A CATAPULT OR TRIBOK?
The catapult balls found during the reconstruction of the market square have a mass of approximately 10 kg. According to several sources, the average mass of balls used in medieval catapults was between 10 and 20 kg. The most powerful catapults, or rather their technological cousins, the counterweighted trebuchets, were able to launch objects with a much greater mass. The data are not entirely consistent, but it is estimated that the maximum launched mass was between 900 and 1300 kg (it is not known to what distance). More recent reconstructions of medieval counterweighted trebuchets were able to launch a maximum of 476 kg at a distance of 80 m.
LOGGING TASKS
1. When did the formation of the stone into a stone ball begin at the latest, if a glacier was involved? To find the answer, you will have to delve into your elementary school geography knowledge. Or browse the Internet. How long time ago, at the latest, could that be? One of the following answers is correct: 10–15 thousand years, 100–150 thousand years, 10–15 million years, 100–150 million years.
2. What is the diameter and mass of the stone ball? Measure or estimate the diameter. Based on the measurements, calculate the approximate mass. 1 cubic decimeter of limestone has a mass of approximately 2.6 kg (1 m3 is therefore 2600 kg). Use the formula for the volume of a sphere: V = 4/3 π r⊃3;, where V = volume and r = radius.
3. Could this stone ball be launched to a distance of at least 80 m with the largest modern reconstruction of a medieval trebuchet?
4. The stone ball has a modern, manmade addition. Look more closely at the surface and report what that is.
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WHY »A THREE MAN BOULDER«?
It comes from the estimation that it takes three adult men to move this size of stone. The term is used by (American) landscape material sellers. They also know Half Man, One Man, Two Man… up to Six Man rock sizes 😊
Viri / References:
eagletimes.com, 2020, The outside story of drumlins and erratics
worldrivers.net, 2020, Sediment transport