What is a pebble

Isar - Kiesel: rocks from the Alps


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Geology and geology

Pebbles of the pre-alpine rivers using the example of the Isar

The Isar's rock load, correctly referred to as rubble or debris, comes from the catchment area of ​​the Isar and its tributaries. So mainly from the Northern Limestone Alps, but also from the Central Alps and the Northern Pre-Alpine Zone.
The Isar rock stock is largely identical to that which is also found in all other rivers between Lech and Inn, as well as in gravel and gravel pits of the Bavarian foothills of the Alps and the Munich gravel plain can be found.
The finds on display come mainly from various gravel banks along the Isar between Bad Tölz and Garching as well as from some gravel pits in the east of Munich.

The Isar

The official source of the Isar is in the Karwendel Mountains, in the Hinterautal at approx. 1200m above sea level at the junction of several mountain streams. After about 280km the Isar flows into the Danube near Deggendorf.
The Isar is one of the last alpine rivers with near-natural sections. Especially on the Upper Isar up to the confluence of the Loisach, but also further along there are (more or less) extensive gravel or gravel fields. An ideal "fund" for stone collectors.
This website deals with the stones of the Isar, Iller, Lech and Inn and their catchment areas: where they come from, how they were created, what they are made of, how old they are, etc.
Photo: Munich; View from the Maximiliansbrücke to the south on a gravel bank, the cable bridge and the Deutsches Museum in the background.



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Colors of pebbles

Isarkiesel (Innkiesel, Lechkiesel etc.) are exclusively rocks from the Alps and the Alpine foothills.
The largest share is made up of pebbles with various shades of gray from almost white to dark gray. Yellowish to brownish-gray colors are also often represented. They are mostly carbonate rocks from the Triassic period.
Colorful, mostly reddish to red stones, were mainly deposited in the Jura.
Gray-green tones indicate Cretaceous sandstones, while black-and-white speckled or banded crystalline stones come from the central Alps. Quartz are usually milky white.



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Main ingredients

the bed load


By far the largest share of bed load from Isar, Inn, Lech and Iller consists of alpine sedimentary rocks from the Triassic period.
There are mainly shallow water limestones and dolomites as well as reef limestone that were formed in the edge areas of a prehistoric sea called Tethys.
They mainly belong to the main dolomite, including slab limestone (Vorkarwendel, Bavarian Prealps), as well as the Wetterstein limestone (mainly from the Karwendel mountains).
The predominant colors are shades of gray in all shades.
See:


Structural features

River pebbles often have characteristic structural features.
The Grain sizes range from very fine-grained (no components recognizable) to coarse-grained, whereby the components can have very similar but also very different sizes.
It is usually easy to see whether the components are crystalline or of sedimentary origin.
Even the material one composition can be quite uniform but also very diverse.
The arrangement the rock components can be irregular, sorted, laminated or layered.
These petrographic features can usually be used to determine the stones.
>> Further explanations and details on structural features



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Origin of the pebbles

The gravel bodies in the upper reaches of the pre-alpine rivers consist mainly of erosion products from the limestone Alps of the respective rivers.
As the transport distance progresses, the grain diameter of the stones increasingly decreases due to the abrasion.
Isar: In the south of Munich, the Isar cuts into the hard Nagelfluh banks and ceiling gravel, which are important suppliers of bed load with regard to the proportion of crystalline rock.
The gravel banks of the Isar as well as the ice age (Pleistocene) gravel of the Munich plain (a "glacifluviatile", thus formed from meltwater rivers, rubble compartments with a high degree of roundness) contain material of the meltwater contents of reclaimed moraines as well as earlier gravel material (the crack and Mindeligzeit) from Isar- and Inn Glacier.
In the last ice ages, the glacier advances reached far into the Alpine foothills.


The origin, especially of the crystalline stones (greenish granite, gneiss, etc.) that come from the Moraines origin, can partly be traced as far as the Engadin (area of ​​St. Moritz) due to the glacial origin and were with the Inn glacier transported.
Sketch of the map showing the origin of the most widely transported rocks:
Transport route of the Inn Glacier

The young Inn in the Engadine:
Moraine on the still young Inn in the Upper Engadin

The gravel body of the Munich Inclined Plane is predominantly in the form of low-terrace gravel from the Wurm Age, only in a few places (e.g. between Berg am Laim and Ismaning) did older high terrace gravel resist the erosion effect of the Wurm Age meltwater floods.
The sketch shows the terminal moraine courses of the maximum extent of the glacier tongues in the Würme Ice Age and the Crack Ice Age as well as the transport route of the moraine material.
Since the expansion of the Isar at the end of the penultimate century and in the course of the last century (e.g. Sylvenstein reservoir), there has been practically no natural entry of bed load any more.


Autumnal inversion weather. The Isar glacier would have given the viewer a similar impression around 15,000 years ago if he had stood on the Seekar summit and looked out into the north-west of the Isar valley. At that time, large areas of the Alpine foothills and the Alpine foothills were covered by the ice masses of a glacier.
(Right part of the panorama picture: view to the northwest, on the right the Geierstein, then the Isar valley near Lenggries, the Blomberg summit can be seen very small in the background, and in the middle, slightly to the left, the Benediktenwand. Left part: view to the southwest towards Jachenau and Isarwinkel.)
Photo taken on October 24, 2012 with pronounced inversion weather conditions (cold below and warm above). The thick fog ends abruptly at an altitude of about 1000m.

Rubble form

The transport of rubble is the cause of the accumulation and depletion of stones of different compositions.
The criteria for this are the strength, weatherability and solubility of the rock material. For this reason, brittle and otherwise easily destructible stones will quickly disappear from the gravel downstream. Only resistant stones provide pebbles that can last for longer periods of time. It is less the hardness than the toughness that determines whether a rubble is formed that is permanent.
A distinction must be made between the two independent parameters, the form of the rock (shape) and the degree of rounding
further in the text, explanations, details on the formation of pebbles
Above: Scree with an almost perfect ellipsoid shape.


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Corridors

(Veins, crevices)
In the case of tectonic stress, e.g. in the vicinity of fault zones, rocks can break, the breaks are called fissures.
If calcareous solutions flow through these, the crevices can be completely filled with calcite in the course of time. Pebbly stones (e.g. red radiolarites) of the Kalkalpin are particularly brittle and therefore often rich in healed fractures. Fissures and passages in the gneiss and slate of the central alpine area are not filled with calcite but with mostly milky, cloudy, greasy quartz.
In the picture a dark gray Alpine limestone from the Middle Triassic period. The crevices are filled with pure, white lime. On the intersecting cleft lines with a slight offset, several temporally staggered fracture and healing phases can be clearly recognized.
These fissures testify to the tectonic processes in the course of the formation of the Alps.



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(Small) tectonics

The Alps were created by the relative north (west) movement of the southern continent Gondwana, or the offshore Adriatic plate towards the Eurasian continental plate.
The resulting deformation structures in the mountains are not only detectable in the large folds and thrusts that extend over many kilometers but are often "frozen" down to the smallest area in the pebbles.
Depending on the depth and temperature at the time of the action of the tectonic forces are

brittle fracture structures (picture above)
or
plastic flow and fold structures (picture below)

formed in the rock.



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Fossils

With a little luck you can find fossils, (Fossils), or find their prints.
In addition to rocks from the Kössen strata, reef limestone from the Upper Triassic, the Allgäu layers (= Lias speckled marl) and some stones from the Helvetic and Molasse are particularly rich in (macro) fossils.
Here as an example (picture left) pebbles with corals (right in the left picture) and mussel shells (left and below)
Age: Trias Limestone of the Northern Limestone Alps

Please refer Further fossil finds in pebbles.



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Fossil plant?

First picture: What looks like a twig is in truth the traces of life of a sediment-eating animal.
Here a plant-like tunnel system of branching feeding passages (chondrites).
Second picture: What looks like a leaf is part of a line of praise (chamber partition) of an ammonite.



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Attachments

Rock material moved by glacier ice (glacial) is called sediment,
Rock material moved as rubble by flowing water (fluviatil).
Isar gravel can be classified to a not inconsiderable proportion as glacifluviatil transported material, i.e. it has experienced both types of transport.
Due to the ice movement and the mutual friction of the moraine material of the inner and ground moraine, the bed load material was scratched, scratched and polished under increased pressure conditions.
While the scratches are no longer recognizable even after the shortest transport route in flowing water, pebbles with concave shapes and carved indentations reveal their origin as material transported by the glacier ice.
They are called Throat attachments designated.



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Calcite crystals

The alpine fissures emerged as tectonic fissures, e.g. in the course of the elevation of the Alps.
In the case of the formation of crevice-like fissure cavities, well-formed minerals could crystallize out of the circulating salt solutions.
In the picture Calcite crystals in a cleft cavity, very much enlarged.



>> Explanations for the map sketch,
Geological structure of the Northern Limestone Alps

Geology (Northern Limestone Alps and Foreland)

In the Alpine region, the Isar crosses for the most part layers of the Triassic (mainly Wetterstein limestone and main dolomite) and Jurassic limestone. The main dolomite takes up most of the area in the northern limestone Alps. Layers of the Cretaceous and Tertiary (molasses) contribute to a subordinate extent to the diversity of the rock load.
A large part of the material transport to the north took place in the ice ages with the Isar glacier as moraine material. The crystalline part of the debris load (mainly gneiss) comes exclusively from the moraine material brought in from glaciers. It contains stones whose origins extend as far as the main Alpine ridge and southwest into the Swiss Alps.



>> Explanations for the map sketch,
Geological structure of the Vorkarwendel

Geology of the Front Karwendel Mountains

A brief description of the rock inventory, the formation and the geological structure of an area section of the Northern Limestone Alps in the catchment area of ​​the Isar.
The Karwendl Mountains are one of the main suppliers of the rocks to be found on the Isar.



>> Explanations of the map sketch, geological structure of the Northern Limestone Alps

Alpine Geology (Western Eastern Alps)


The three main ceiling systems of the Alps, that Helvetikum (green) and Penninic (red) collectively referred to as Westalpin, as well as that Eastern Alpine (yellow) originally represent separate educational spaces. After their formation, these were pushed together very strongly in the course of the continental movements and stacked on top of one another.
The paleogeographical unit of the Helveticum on the northern edge of the Tethys (in the area of ​​the European continental plate) came to be at the bottom.
The upper blanket of the Eastern Alps, Austroalpin, was furthest south on the edge of the microcontinent Apulia off Africa.
See:



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Stratigraphy (sequence of layers)

The pebbles described on the Sedimentary Rocks page from the Northern Limestone Alps and areas to the north were deposited in the Mesozoic and Cenozoic.
As far as possible, they are listed in chronological order.
The geological terms used for the chronological breakdown (e.g. Tertiary, Triassic, Scythian, Anise, Ladin, Rät, etc.) as well as the rough chronological classification can be found in the .


Rock pictures, descriptions and rock identification

The descriptions of the river pebbles are divided over the pages Sedimentary rocks and Crystalline stones .


The

Sedimentary or deposit rocks

come mainly from the Northern Limestone Alps from the catchment area of ​​the Isar and are mostly limestone.



The

Crystalline or metamorphic rocks

come mainly from the Central Alps and are mostly gneiss. They were brought in with the glacier ice and make up about 15% of the gravel material.


Classification note:

The determination of a large number of characteristic Alpine stones is relatively easy, but the correct assignment of many other river pebbles is not so easy to carry out.
The reason for this is that at different times similar deposit or formation conditions prevailed, which therefore led to the formation of very similar rocks. This applies, for example, to the large group of more or less "gray Alpine limestone" from the Triassic.
For the exact determination or the stratigraphic allocation, petrographic and paleontological methods such as micro-palaeontological evaluation of the fossil content, microscopic or polarization microscopic examination of the mineral stock as well as chemical analyzes are required.

Illustrations:

The average size of the pebbles shown is approx. 5 cm (+/- a few cm).

Frequency:

very often (without searching to find it at almost every location)
often (to be found on every excursion)
occasionally (only through careful search, maybe not to be found on every collecting tour)
Rare (Perhaps to be found on every 5th or 10th excursion or only at certain locations, specific search necessary)
very rare (belongs to the rarities, perhaps only known in a few finds, but offers an incentive to collect)



© www.isar-kiesel.de - This website is used no cookies, it will no user data saved. For explanations, see data protection notice.