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The Irongate Project

The Geology of the Irongate Region

Version 2.1
by Denis Tetreault

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.)

Kurst Islands

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.

 A side note on the science of this page (or lack thereof)....

As a geologist in real-life, I am compelled to point out a few things. Limestone gives a great landscape filled with caves, ripe for adventuring, but on its own contains little in the way of gems and native metals which, according to the WoGH Gazetteer, are supposed to be present. Therefore I've used the presence of scattered volcanic intrusions to supply the source material for these resources. The scattered nature of individual igneous bodies produced by volcanics enables mines to be hard-to-find and isolated. While all of these minerals can be found in these general rock types, each depends on wildly different variations in the chemistry of the original source rocks. In other words, it would be impossible to find them all present in the same location. Also, a true hot-spot, such as that which produced the Hawaiian Islands chain, produces mafic and ultramafic rocks (mainly dark rocks such as basalt and peridotite). Volcanoes originating along collisional continental margins (where major mountain-building occurs with the subduction and melting of material) produce felsic magmas (which are the source of pegmatites and many of the gems found there-in). And from the World of Greyhawk maps, we don't appear to be near any sort of plate boundary or mountain range (not that those maps resemble in any way a geologically-sound construct). Therefore, while it is all volcanic material, they are generally mutually exclusive.Under special circumstances, it is possible to compositionally differentiate ultramafic magma to the point of producing a felsic dyke, but in practice, this rarely happens to such an extreme degree. But without those in the region, the mineral resources would be rather limited. Hence, I call upon the magical nature of the fantasy world to accomplish this. ;-) Relate the hot spot to a confluence of portals to the Elemental Planes of Fire and of Earth and just about anything is possible. Just don't waste your time trying to find a real-world gem mine using the descriptions here-in. ;-)

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This page last modified on August 18, 2009