Ancient Environments of Deposition

The Interactive map

This interactive map is, like most of the other maps on this site, built in layers which can individually be switched on and off. The map can be zoomed in and out by use of the + and – buttons in the top left, or by the use of a mouse wheel.

The legend for all layers is found by moving the mouse pointer over the icon in the top right. Layers lie one on top of another, so the lower layers will be overlain by those higher in the legend. The map is displayed within it's own frame and can be moved by dragging with the left mouse button depressed.

Clicking anywhere on the map will raise a popup box that describes the layers active at that point, in sequence, from the topmost to the lowest.

The layers on the map show, alternately,

  • Numbered Layers - the sediment that was deposited, the environment, and the area covered by that environment at that time. The extent of each of these is partly conjecture - we have no idea exactly how far these differing environments may have extended, apart from the current position of the remaining rocks.
  • A few of the numbered layers are structural - the basin faults ( layer 6a, the Coomnacronia, Killarney-Mallow, and Dunmanus-Castletown faults) and the blocks of upstanding basement rocks (layer 6b, the Sheep's Head and Glandore Highs)
  • The unnumbered layers between the numbered layers show the rocks that are left now, as outcrops, from the preceding layer's sediment. They are marked as 'Now', as opposed to 'Then'. This is where things go a bit awry, because the modern day outcrops that are portrayed are in reality in a landscape that has undergone quite severe faulting and folding; and yet here we are displaying them on a map on which the folding and faulting has been 'undone'. Think of this as an exercise in aiding undertsanding of the complex sequences of events, and not a picture of reality. We are playing with time.
  • Because the unnumbered layers represent current day outcrops, they do not overlap. So they may occur in the popup box when you click with the mouse button, if they are active at that point. The layers of sediment are described with Formation Name, Symbol, and Sediment, because they are sediments; the modern day outcrops are described with Formation Name, Symbol, and Lithology, because they are rocks.
  • Locations are locations described in the GSI Geological Descriptions. Clicking on these locations should give a listing of those formations that occur at that point on the landscape. These do extend outside West Cork, and near the edge of the map things get a bit hazy
  • Finally, the coastline. The outline of the modern coastline, though stretched in a NS direction, is only for relating the areas of depostion to modern day West Cork. That coastline did not exist at the time we are talking about and as the encroaching marine environment shows in the layers, the relationship between sea and land was very different then to what it is now.

Click here to go to the Interactive Map

The Area of the Map

This map represents an exercise in time travel, and is fraught with problems. But it is a fun exercise.

The coastline portrays south west Ireland - our focus is on West Cork, but this has been extended because of the association with the Munster Basin - but south west Ireland of about 380 million years ago. This is a plainspastic reconstruction, that is, the crushing and shortening effect of folding and faulting in the intervening years has been undone. So the map is stretched by about 150% in a NS direction, as it was before plate movements put the squeeze on.

The map has been designed to show the progressive changes in environment - subtle though they were most of the time - that occurred over the roughly 80 million years, from the Mid Devonian through to the Mid to Late Carboniferous periods, during which the rocks that currently form the bedrock of West Cork were being deposited as sediment.

The map initially displays the position of landscape elements as the deposition of the sediments that were to become the rocks of West Cork was just starting. The basement rocks were probably of similar type and deposited in a similar environment to what we see as the deposition sequence starts. To the north note the high ground - mountains in the area of what is now Dingle Bay. Fans of rock fragments and coarse sand and gravel where flood water and rivers issue out of the mountains heading south, and underneath that, a major fault where the earth's crust lying to the south is gradually sinking.

To east and west the substantial blocks of granite (the one in the west is conjectural based on gravity measuremnts) stabilised the crust, and so the subsidence of the basin did not extend here.

Note particularly the northward movement of the coastline and the change from terrestrial environments to coastal and then marine environment. Again, this map is just a representation in very broad outline of what we think happened.

How to use this map

With all layers switched off as the map is when first displayed, switch on the numbered layers starting at 1. You can switch on the intervening layers as you progress. Clicking on the areas that display on the map will give descriptions of the environment, and the names and geological symbols of the formations that they gave rise to. All the formations grouped together under one name are considered to be more or less from the same time period. Thus moving up the numbered layers in sequence will give a view of the environments of deposition progressing through time. The popup boxes that display when you click on the map will show the layers trhat are active at that point. To read these in chronological order, start at the bottom.

Having reached the top - layer 18 - and investigated all the different environments that occurred at different times in the different places, now switch off all the numbered layers and leave all the others on. The result will be a geological map of the surface outcrops of the main formations.

How this map was made

The map has been generated, using a program called QGIS2Web, from an Open Source GIS called QGIS. This software is constantly being improved and, like all open source products, is highly efficient and free. The layers have all originated from data made freely available to the public domain. There are some inconsistencies in the map, arising partly through the different sources the data came from not being in alignment either with accuracy, completeness or definition.

Read more about how these maps are made and where the data comes from.

Click here to centre the map in the screen

Click here to centre the map in the screen