Archive for the ‘Geological’ Category

2012 TDFC Weekend Excursion – Caithness

Monday, May 28th, 2012
The drifts of delicate spring squill on Dunnet Head were a highlight of our walk. Continue reading “2012 TDFC Weekend Excursion — Caithness” »

Jurassic Foreshore at Helmsdale

Saturday, February 26th, 2011

A day on a Jurassic Beach.

2539_TDFC_group_SFWClick image to view

From south of Helmsdale north to the Ord of Caithness the rocky foreshore is a wave-cut platform of Jurassic (Kimmeridgian) rocks.  TDFC members walked north from Helmsdale looking for fossils, for birds and other wildlife, and looking in detail at the silty or sandy bedding, or the breccias and conglomerates standing above them.

The Helmsdale Fault is present just on-shore showing a steep scarp slope facing the sea,

2569_HFaultPlatform_ed-1SFWClick image to view

and much evidence in the conglomerate or the siltstone (1st picture above) deposited below the fault by eroded and falling detritus (like the ‘fallen sea stack’ below).

FallenStackClick image to viewKimmeridge silts distorted below fallen cobbles of Old Red Sandstone

The Helmsdale fault is associated with the Great Glen Fault , running from near Dingwall, through the Struie Ridge, to this shoreline and out to sea below the Ord of Caithness.

The Kimmeridgian strata sloping up to the rocky wave-cut platform appear curved, as below.

Curving Jurassic conglomerate bedsClick image to view

This is possibly due to strains imposed by being near to the moving fault.  As these beds slope eastwards down below the Moray Firth and the North Sea they become the source rock for 95% of the oil found offshore.  Around the world rocks of this age are now home to many dinosaurs, large and small, marine and terrestrial.

Many examples of Jurassic ‘wood’ were found, some brown lithified rock and some carbon-black appearing objects embedded in sandy siltstones.

Several fine coral assemblages were found, and many examples of crushed bi-valves in a silty matrix.  Here are a fine coral and an ammonite.

A fossil CoralClick image to view AmmonitefossilClick image to view

Thankfully this was a fine, sunny, cool February day for looking at these rocks and birds like a single early Gannet.  And a fine lunch spot.

LunchClick image to view

And, other wildlife was found (Anguis fragilis):-

Slow-worm on groundClick image to view

Northwest Geopark

Thursday, February 3rd, 2011

Donald Fisher spoke to us about Geology in general, British geological history, and Northwest Highland Scottish geology in particular.  The main import was to explain why the Northwest Geopark is important in geological terms.

Donald Fisher spent some time as the Highland Region Geologist before setting up a Geological Consultancy business.  He lives in Scourie, at the heart of the Northwest Geopark for which he is the Geological Ranger.

The lecture began by asking how well we personally recalled times only recently passed, like previous past hours, previous past years, or previous past generations that we just might have a notion of.  Then we were asked about our understanding the meaning of 10,000 years ago, 50,000 years ago, 1,000,000 ago, or more.

We were assured that while geologists speak in terms of many millions of years past and processes requiring millions of years to elapse no one, not even a geologist can accurately understand these time periods.  The point being that we must not let these large periods of time be a barrier to discussing and understanding what has happened to the surface of our planet over geological time.

Before entering into the geological explanation of how Britain became what it is today Donald outlined the three major rock types, Sedimentary, Igneous, and Metamorphic, their origins and general relative hardness.  The landscape we now see reflects these underlying rocks.

In very general terms England is underlain by sedimentary rock formations and displays a gently rolling surface.

Scotland, with more metamorphic and igneous rocks at its surface, displays generally steeper and higher landforms.  Where sedimentary rocks lie below the surface in Scotland, the Central Belt, the landforms are more undulating than mountainous.

Guided by this explanation observation of the Geological Map of Britain began to make more sense.

It can never be easy within about one hour to explain several billion years of geological history, but Donald Fisher used as a target date “When England Met Scotland” and explained movements of tectonic plates (mini-continents) from very far south of the equator to a point, still below the equator, when the landmass Scotland was attached to, Laurentia (Canada and north USA), collided with the landmass England was attached to, Avalonia (Europe, except northern Scandinavia).

Since that collision England and Scotland seem tightly bound – geologically.  And, they have moved much further northward.

The stress of the collision between “England” and “Scotland” caused disruption which formed one of the main features of interest today in the Northwest Geopark – namely the Moine Thrust.  A very large slice of old metamorphic rock, the Moine Schist, was pushed,  or thrust, by this collision northwestward over the younger rocks of the Torridonian Sandstone.

Understanding what this thrust was and how it came to be was instrumental to the birth of Structural Geology.  So now the Moine Thrust area is rightly famous worldwide.

Other geological features in the Geopark include the Lewisian Gneiss, nearly the oldest rock occurring on the surface of the planet.

Apart from providing an explanation for ‘thrusting’ other process are well displayed in the Geopark, such as glacial erosion which has removed masses of rock from between our favourite West Sutherland hills and left us with the photogenic Suilven, Quinag and other peaks.

Our individual observations of ‘three score and ten’ years are quite minute when compared with the immense time periods our unique Geopark landscape displays.

We thank Donald Fisher for the concise addition to our understanding of Highland landforms and geology.