Millions of years ago the Irongate region was a shallow tropical sea, where thick sequences of limestone were deposited. During this time a geologic "hot spot" passed beneath the crust of the region creating clusters of small volcanic islands, leaving behind a string of short-lived volcanic intrusions, lava flows and other deposits, including hydrothermal vents. The exceptionally deep source of the hotspot enabled certain unusual rocks (such as dunite and peridotite) to be emplaced along with the more common basalts. The hydrothermal deposits include uncountable quartz veins criss-crossing the region. In some areas the heat of these intusions metamorphosed some of the surrounding rocks. Late stage magmatic differentiation later injected pegmatite dykes (coarse-grained granitic rocks) into the area. As time marched on, the volcanic mountains eroded away, releasing iron (and other elements), producing deposits of oolitic iron and placer deposits of insoluble metals eroding from the intruded rocks. More limestone was deposited on top of all this. Eventually the area was uplifted out of the sea and the sub-tropical environment quickly began eroding the limestone. There is now little evidence of the original volcanic mountains, except for the scattered exposures of igneous rocks imbedded within the limestone.
The karstic landscape that now dominates the region is a product of
the dissolution of the limestone. Above ground this results in very
steep-sided,
flat-bottomed valleys incising the entire plateau in a meshwork of
narrow
valleys of bewildering complexity. Detailed mapping is a virtual
impossibility.
Movement through the area is difficult, and travel is limited to the
valleys.
This results in highly meandering roads with relatively short
line-of-site
distances (a bandit's paradise). Even if one climbs the almost vertical
valley walls, they find the "plateau level" to be very irregular and
rolling,
relatively densely wooded, and fraught with numerous sinkholes and
otherwise rocky
terrain. Highly eroded pinnacles of limestone stand as half-submerged
rock towers and mesas off the south coast of the Dragonshead Peninsula,
forming the Kurst Island chain.
A
view along a typically narrow, steep-sided valley in the Headlands
region.
The subtropical climate erodes deeply into the limestone terrain
producing
high cliffs and numerous caves. (Original image of karst landforms in
Shaanxi
Province, China, from the photo-archives of the Global
Karst Correlation Project. Used without permission.)
The Kurst Islands off the south coast of the Dragonshead Peninsula
are formed by submerged limestone karst landscape. While some are
larger, most of the islands are fairly small, and often difficult to
get onto. These rugged islands are riddled with caves. (Original image
of karst islands
in the Andaman Sea, used without permission.)
While the deep tropical chemical weathering of rocks such as
granites and basalts (lava flows) produce quite rich and thick soils
(which are locally possible over the few areas in the region that have
those types of rocks), the dissolution of limestone leaves behind
little soil. Thus the hills and plateaus have poor soil development,
highly karstified and irregular exposed rock outcrops, and thinner
(while still fairly significant) forests then one might expect based on
climate alone. The steep cliffs and hillsides cannot hold any soil as
the frequent rains wash everything into the valleys. The larger valleys
therefore do have much better soils then the hills, resulting in much
denser vegetation and patches of near-impenetrable forests. This richer
soil also serves as reasonably good farming soil. As a result, numerous
small
farms have been established close to Irongate where the city forces can
provide some measure of protection from the dangers of the wild
lands. It should be noted, however, that tropical soils in general are
fairly poor compared to temperate soils, as most of the nutrients are
caught up in the lush vegetation and not in the deeply weathered soil
profile. Remove the native tropical vegetation, and bio-productivity
drops precipitously. The farming here does produce supplemental
foodstuffs for Irongate, but it is by no means a cornucopia of
abundance.
Below ground, the dissolution of the limestones have resulted in the formation of uncountable cavern complexes of every size over the entire region. Many of these caves open up into the valley walls and cliffs, providing refuge for numerous creatures, both intelligent and unintelligent. Some of the cave mouths disgorge streams and waterfalls, other caves and sinkholes may swallow streams. Some of the caves penetrate much deeper into the Underdark.
Because of the complex interplay of rock types, there are numerous types of natural resources found. The oolitic iron deposits (hematite) are scattered as bands and layers within the limestones throughout the area, though are more common in the Iron Hills region. As a result, most of the currently active mines are to the east of Irongate, but small mines (many abandoned) dot the entire area. These provide the source of iron so famous to the area, and have been mined by dwarves for a very long time. As they mine along the hematite bands, they often intersect natural caverns. Some of the hydrothermal quartz veins permeating the rocks contain precious metals such as gold and silver. Finding these "mother load" veins is very difficult (only a few have been found, and all of those by dwarves), so much of the gold being removed is "placer" gold (either panned from the rivers, or mined from "paleo-placers" as sandstone or conglomerate layers within the rocks of the cliffs). Not surprisingly, the dwarves prefer to find and excavate the paleo-placers, and the source veins, rather than muck about in the water.
The classically sought-after iron, gold, and silver are not the only richest found in the region. The basalts sometimes produce obsidian, which has a variety of uses (including spear points and arrow heads by very early inhabitants and some of the more primitive present-day humanoids). Within the hydrothermal vein deposits can be found several types of semi-precious quartz (including rock crystal, agate, amethyst, chalcedony, carnelian, and citrine varieties). Within the pegmatite veins and bodies are many other types of gems, including tourmaline, quartz , beryl (including aquamarine and emerald), topaz, moonstone, and zircon. The high temperature peridotite deposits not only sometimes produce high quality olivine (known as peridot), but in the Headlands region they are associated with very rare platinum deposits (found as placer deposits in the nearby streams... the source still remains undiscovered), and in the Iron Hills region their high temperature of intrusion metamorphosed the surrounding limestone into very high quality marble. This marble is mined by the dwarves of the area and shipped out as building stone. Parts of the city of Irongate are constructed of this high quality Iron Hills marble. Rumor has it that a few diamonds have been found in some streams deep within the Headlands. Stories abound about a lost diamond-bearing kimberlite deposit. Few believe it.
Minerals that would probably not be found in this area
include
amber, corundum (ruby and sapphire), garnet, turquoise, tiger-eye,
jade,
jet, and coal. Some coal is imported, but most smelting is done with
charcoal
made from the local trees.
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