I think your translation may be a little off there joćo paulo.
I think it may be hundreds of feet, not hundreds of miles.
From the following website:
http://www.livescience.com/technolog...rth_drill.html
But still, cool information know.The Earth's radius is about 4,000 miles (6,400 kilometers). The main layers of its interior are in descending order: crust, mantle and core.
The crust thickness averages about 18 miles (30 kilometers) under the continents, but is only about 3 miles (5 kilometers) under the oceans. It is light and brittle and can break. In fact it's fractured into more than a dozen major plates and several minor ones. It is where most earthquakes originate.
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The link that JP gives in his post actually says the island is 19,000 square kilometers (~12,000 square miles).Originally Posted by Redrobes
From http://en.wikipedia.org/wiki/Bananal_IslandThe Bananal Island covers an area of 19,162.25 km², twice the size of Lebanon or Jamaica. It is the largest fluvial island in the world being 350 km long and 55 km wide.
That seems pretty large to me, but, granted, that is a rarity. The largest one I know of is about a square mile and most are a few feet.
I am embarrassed...
I apologize to everyone.
In this map of the state of Tocantins you can see the size of the absurd that did.
The island in question is in the lower left corner.
Last edited by joćo paulo; 01-03-2009 at 09:47 PM.
Very interesting thread! Given that I'm Minnesotan I well acquainted with lakes, we have thousands of them gouged out by ice sheets during the last ice age. An observation I've made is that besides lots of lakes the glaciers left the local-scale drainage patterns a bit messed up and erratic in places with stream-water going short distances zig-zagging from lake to lake until a substantial river with a flood-plain is reached.
I guess the more modern the geology the more weird its going to look. Even with extremely recent geology the water should still adhere to these general principles because water moves so fast in comparison to changes in rock. Your area showing the zigzags of streams all finding a new and comparatively fresh path to the sea should convince us all that it has to be this way.
That is very true. The reason for the messed up drainage patterns is that the ice sheets left a lot of debris, resulting in lots of hills and ridges, and dug out big holes that became lakes. It's actually quite a beautiful landscape
Geomorphology – i.e. the geographical processes that affect the landscape are of particular interest to me. I have just found this post and would like to add a few points that hopefully those looking for more information will find useful / informative.
Springs
For this statement to be true you really need to divide water into two categories:1. Water flows downhill and it does so in the direction of steepest decent.
2. Rivers originally start with water which has fallen from rain.
Ground Water
Surface water
For surface water like streams and rivers both of these statements are obviously true.
For ground water the situation is more complicated and your discussion on springs (I feel) needs a little clarification.
There is the type of spring as you rightly say is the source where an underground river or groundwater emerges under the effect of gravity.
In Southern England the usual meaning of spring is where ground water emerges for the first time at the surface and isn’t reliant on gravity.
This effect is reliant on bands of permeable and impermeable rocks or clay. Rain falling on the permeable rock seeps in as you have said until it reaches the impermeable layer. Here the weight of the rock above forces the water upwards to where the two different layers meet at the surface. This is where springs form. If this happens to be at the top of a hill ponds can form in any small hollow. If it happens on the side of a hill a stream forms and begins to fall down the slope.
The Weald of Kent is a good example of small ponds on hill tops and spring lines on ridges.
Rivers
River outflows:
I disagree with this statement, lakes by definition cannot form on a permeable surface. Where a river meets a permeable rock a sink hole will usually form. Where it meets a softer rock a waterfall is likely.2. Seep into the ground (i.e. overflow into the ground)
Lakes
It seems a very obvious point, so obvious its rarely thought about but what has caused the placement of a lake. A hole in the ground is too simple because the only type of lake a river on its own produces is an Ox-bow lake that form through meanders and are limited in size and depth to the river that formed them.Only in a basin will a lake possibly form
So we need other mechanisms to form lakes, or more accurately form the hollows in the ground AND this is the crutch point some way of damming the hole so the water can’t escape.
The large lakes for example those in the African rift valley are formed by plate tectonics. This cause the “natural depression” mentioned earlier that can fill with water. Other examples of a rift valley lake is Lake Baikal which contains 20% of the worlds fresh water.
The Scottish lochs in the Great Glen have been caused by glacial erosion (see more below) along an old transform fault.
Glaciation is a whole different kettle of ball games and opens up a huge range of lake types. Glaciers are capable of gouging massive scars in the landscape but again it requires some fairly specific stages for a lake to form. From the top of my head glaciation forms four types of lakes
Corries / Cirques
These form in mountain regions normally on a north facing slope and mark the point a glacier once started, the glacier grinds a basin with a lip at the front which now retains the water.
Usually small in size but VERY numerous in most mountain regions.
Ribbon lakes
A glacier traveling through the base of a valley will erode uniformly, something tat will not form a lake. However an area of softer rock will leave a hole which can then be dammed by end moraine (lots of rock left when the glacier retreats.
Fairly common in upland regions and range from a modest size
Fjords
Fjords are in very simple terms a ribbon lake that has been flooded by rising sea levels. Can be very numerous at the right latitude but obviously are by definition on the coast.
Knock and Lochan
A unique glacial landscape and one that forms areas with huge numbers of small lakes. I will let you read about it yourself but again glacial scouring leaves small hills (knocks) and small lakes (lochens). The drainage in such areas can be very complicated so feel free to have crazy rivers and lots of boggy ground.
One final type is so rare and unstable but deserves a mention for accuracy are the lakes that form when a glacier cuts across a river valley. Meltwater trapped behind the ice builds up and has in the past formed massive lakes. The effect when the dam burst so to speak is also quite spectacular. Read more about it here - http://www.glaciallakemissoula.org/ but for any GM looking for a 2000ft wall of ice or the geographer looking for how something like the Columbia River Gorge here is a possible explanation.
Volcanoes
Crater lakes are worth remembering and can be quite spectacular. In real life and also on a map.
Gorges
Getting tired now like I’m sure you are reading all this but the important thing if you want a gorge is that unless sea levels change rivers can’t cut through terrain on the lower reaches of the river, it will just go around. Gorges, rills and valleys are possible at the upper reaches of the river near the source due to the river cutting into the terrain but not at the lower reaches unless…
What is more common in forming gorges is that the land is uplifted in some way and the river continues its path and as the land pushes up it appears to cut through the land.
I’m sure I have forgotten something but this is the kind of detail that interests me. Any mistakes feel free to point out.
Last edited by Korba; 03-31-2009 at 08:47 AM. Reason: fixed broken link
yeah, canada has a crapload of lakes, mostly in the norther regions.
These lakes (in the band you mention above) are generally caused by glacial advance and retreat... the glaciers essentially scrape off the land as they advance, and as they retreat, they dump everything they scraped up and move backwards... this creates large holes where the softer rocks wear away faster... but also the deposits the glacier makes can create lakes as well. heres an example of what we mean by a lot of lakes, this is near winnipeg, but areas all along canada have similar lakes.
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That looks like a lot of fun, fishing, kayaking, etc.
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Yeah, the only problem is that there is more wildlife to worry about than just the fish,... Namely the number of bugs that are produced in those conditions. You know you're camping trip is going to go poorly when the family dog gets carried off by the black flies.
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