What a Whopper!!
This is my first earth cache. Its situated at Brockholes Trust nature reserve Just off Junction 31 of the M6.
There are lots of different types of glacial erratics at Brockholes Trust. but this is by far the largest and most obvious of these erratic boulders to be found. Its just off the car park so its easy access for all.
If you are interested in learning to recognise different types of rocks, there is an excellent display of rock samples in the visitor village.
The boulder was transported from the Lake District or Scotland, which could only have been done by ice, and its surface has been smoothed by glacial and fluvial processes. This boulder was found in Lower Brockholes gravel pit, just a few hundred metres away. Erratic boulders, whose place of origin can be identified, enable ice-sheet and glacier movements to be reconstructed.
No rocks of this type, an igneous rock called granodiorite, are exposed at the surface within the river Ribble catchment area and we can be absolutely certain that it cannot have washed in from upstream and must therefore be an erratic. It probably achieved its final polish in the bed of the river.
This large erratic boulder of an igneous rock called granodiorite, from the gravel pit to the west of the M6. It weighs about 2.5 tonne. Most glacial erratics at Brockholes are much smaller.
If you look closely at the rock you will see it contains three major minerals, quartz, feldspar and dark iron-magnesium minerals. Careful examination of this boulder reveals small dark lines which were probably caused by pressure changes while the minerals were crystallising from the molten magma.
Above - A closer view of the rock, which contains three major minerals, quartz, feldspar and dark iron-magnesium minerals using a 2p coin for scale
Above - Careful examination of this boulder reveals lines of dark minerals which were probably caused by pressure changes while the minerals were crystallising from the molten magma,
The Map shows the main ice-sheet flow directions and source locations for glacial erratic boulders in this area
Rocks can be divided into three main classes:
igneous, sedimentary and metamorphic.
Igneous rocks
Igneous rocks are produced at depth within the earth under conditions of great heat and/or pressure. If the molten rock cools slowly within the earth’s crust it is described as plutonic and is characterised by large crystals. Granite is a typical plutonic igneous rock.
Granodiorite is within the granitic suite of igneous rocks with crystal size similar to that of granite. The relative proportion of calcium, sodium and potassium silicates differs from those in a true granite.
Sedimentary rocks
All rocks at the earth’s surface are exposed to the action of weathering. Rain, snow, frost, wind and the action of waves along the shore, attack the rocks, causing them to be broken down in to smaller fragments.
In some cases the rocks can even be dissolved by the weak acid in rainfall, produced by dissolving Carbon dioxide (CO2) in rainwater in the atmosphere. These fragments of rock may then be moved by agents of erosion, rivers, the wind, the sea etc and redeposited, usually under water, but sometimes in sand dunes.
Over long periods of time, these layers of sediment may become new rocks – sandstone if they are mainly composed of sand grains or limestone if they are made of shelly fragments or the remains of marine animals such as corals.
An important difference when trying to identify rock types is to decide whether the rock is made of separate fragments which have become ‘stuck together’ or whether the rock has crystals which have grown from molten rock and are interlocked.
Metamorphic rock
When any rock, igneous or sedimentary, is subjected to high temperature and pressure its mineralogy will be altered by destruction of old and formation of new constituent minerals. Deep burial and involvement in mountain-building processes lead to metamorphism.
Metamorphic rocks are the most complicated of all rocks because the source rock may be of any type whatsoever and there is an unlimited variety of pressure temperature histories that the rock may have been subjected to. Metamorphic rocks may even have been re-metamorphosed by a subsequent and unrelated temperature pressure event. High grade metamorphism occurs where crustal rocks are buried beneath mountain ranges such as the Andes in South America.
A classic sequence of metamorphism is mudstone being compressed until it becomes slate. Further metaporphism would change the slate into phyllite, then into schist and finally into gneiss. If further heat and pressure affect the gneiss, it melts and beomes an igneous magma, cooling to form granite. This is part of what is known as the Rock Cycle, where rock is continually created, eroded, deposited, buried, metamorphosed and created again as new rock.
One well known metamorphic rock is marble, formed by re-crystallisation of limestone. One would describe the alteration of limestone to marble as low grade metamorphism because the temperatures and pressures involved have not been very high. This kind of metamorphism can happen where limestone is ‘baked’ by being near a volcanic or igneous heat source. Quartzite is another low grade metamorphic rock, consisting of crystalline silica. Temperature and pressure conditions have resulted in the sand grains and silica cement being recrystallised.
To claim a find for this cache, you can use information from the cache page and your observations at GZ. Then answer the following questions:
Please send your answers via message centre or email. I assure you I will read all your answers and logs and will reply to each one.
Please do not include answers in your log.
1) Describe the texture of the rock and why do you think it has this texture?
2) What type of erratic boulder is this?
3) Where was the boulder originally from and how did it travel?
4) It was found locally, but where was it found?
5) Estimate its size: Height - Width - depth
6) There are two posts nearby a red metal one and a wooden post. These can be seen on the photo of the boulder. What is written on the top of the wooden post?
7) Optional task: feel free to add any photographs of your visit. no spoilers please.
Note: There is a car parking charge on the Brockholes car park :
April 1st – October 31st £5.00 all day parking
November 1st – March 31st £3,00 all day parking
I have added waypoints for free parking spots - 0.5 miles away