"Geology Class" Why does the professor mention the spokes of a wheel?

Narrator: Listen to part of a lecture in a geology class.
Professor:
Okay, today we’re going to discuss the four major types of drainage patterns. I trust you’ve already read  Q6
the chapter so you’ll recal thata drainage pattern isle arrangement of channels that carry water in an  Q7
area. And these patterns can be very distinctive since they’re determined by the climate, the
topography, and the composition of the rock that underlies the formations. So, consequently, we can see that a
drainage pattern is really a good visual summary of the characteristics of a particular region, both
geologically and climactically. In other words, when we look at drainage patterns, we can draw conclusions
about the structural formation and relief of the land as well as the climate.
   Now all drainage systems are composed of an interconnected network of streams, and, when we
view them together, they form distinctive patterns. Although there are at least seven identifiable kinds of
drainage patterns, for our purposes, we’re going to limit our study to the four major types. Probably the
most familiar pattern is the dendritic drainage pattern.
   This is a stream that looks like the branches of a tree. Here’s an example of a dendritic pattern As
you can see, it’s similar to many systems in nature. In addition to the structure of a tree, it also  Q8
resembles the human circulation system, This is a very efficient drainage system because the overall length of
any one branch is fairly short, and there are many branches, so that allows the water to flow quickly and
efficiently from the source or sources.
   Okay, let’s look at the next example.
   This drainage pattern is referred to as a radial pattern. Notice how the streams flow from a central
point. This is usually a high mountain, or a volcano. It kind of looks like the spokes that radiate out from  Q9
the hubof a wheel. When we see a radial pattern, we know that the area has experienced uplift and that
the direction of the drainage is down the slopes of a relatively isolated central point.
   Going back to the dendritic for a moment. The pattern is determined by the direction of the slope of
the land, but it, uh, the streams flow in more or less the same direction, and.., so it’s unlike the radial
that had multiple directions of flow from the highest point.
   Now this pattern is very different from either the dendritic or the radial.
   This is called a rectangular pattern, and I think you can see why. Just look at all of those right-angle
turns. The rectangle pattern is typical of a landscape that’s been formed by fractured joints and faults.
And because this broken rock is eroded more easily than unbroken rock, stream beds are carved along
the jointed bedrock.
   Finally we have the trellis pattern. And here in this example, you can see quite clearly how the
tributaries of an almost parallel structure drain into valleys and.., and form the appearance of a garden  Q10
trellis. This pattern forms in areas where there are alternating bands of variable resistance, and by that
I mean that the bands of rock that are very strong and resistant to erosion alternate with bands of rock
that are weak and easily eroded. This often happens when a horizontal plain folds and outcroppings
appear.
   So, as I said, as a whole, these patterns are dictated by the structure and relief of the land.
The kinds of rocks on which the streams are developed, the structural pattern of the folds, uh, faults,
and... uplift will usually determine a drainage system. However, I should also mention that drainage
patterns can occasionally appear to be, well, out of sync with the landscape. And this can happen when
a stream flows over older structures that have been uncovered by erosion or... or when a stream
keeps its original drainage system when rocks are uplifted. So when that happens, the pattern appears
to be contrary to the expected course of the stream. But I’m interested in your understanding the basic  Q11
drainage systems. So I don’t plan to trick you with test questions about exceptional patterns, but I
expect you to know that exceptions to the patterns can occur when geological events influence them.