Earth Features and Their Meaning / An Introduction to Geology for the Student and the General Reader

by William Herbert Hobbs

Fig. 492.—Sketch map of Western Scotland and the Inner Hebrides to show location of some points of special geological interest.

It is from Oban, the “Charing Cross of the Highlands”, that one should start out upon the summer steamers in order to reach both Skye and Staffa, the latter with fine basaltic columns ([463]), and Fingal’s Cave. In sailing to Skye one passes upon either shore of the narrow fjords many relics left in the dissection of volcanoes ([139-143] and Sir A. Geikie, Ancient Volcanoes of Great Britain, Vol. II); also rocky islands and skerries marking submergence ([252]), and the coast terraces which register a later uplift ([250]). Skye is a complex of many intrusive and volcanic rocks of such markedly different colors as to appear as tints in the landscape. In the Cuchillin Hills of dark green rises the massive gabbro ([462]) cut by cirques into the jagged pinnacles of horns and comb ridges ([373]); while lower down and to the east are rounded domes of rhyolite ([463]) abraded beneath the glaciers and of a delicate salmon tint. Still lower and to the westward are flat mesas composed of horizontal layers of black basalt under a rich carpeting of the brightest verdure. Eastward across the channel are seen the purplish walls of an ancient sandstone. The jagged gabbro core of the island thus represents a fretted upland ([372]) and is now the training ground of the Alpinist (Abraham, Rock Climbing in Skye, Longmans, London, 1908), while nestled in one of the bottoms of a U-valley is Loch Coruisk, a typical rock-basin lake ([412]), its shores of hard rock planed and scored.

From Skye we may go to study the remarkable thrusts ([45]) on the north shore of Loch Maree, a marked lineament, and one directed at right angles to that on the course of the Caledonian Canal connecting Loch Linne with Loch Ness. This northeast wall of Loch Maree is a strikingly rectilinear fault represented by an escarpment, up which we climb to find at the top the crushed and fluted thrust planes of movement dipping southeastward at a flat angle. Here also are beautiful rock-basin lakes, lying in hollows molded beneath the continental glacier. On our way from Skye we have passed up Loch Carron, a sea loch or fjord ([252]), and along the strath at its head known as Strathcarron ([428]).

Returning now to Oban, it is but a short trip by steamer up Loch Linne to Fort William along the striking lineament ([226]) which continues to Loch Ness and beyond ([Fig. 492]), and thence by rail to Glen Roy and the neighboring glens of Lochaber ([322-325]).

From Paris as a starting point, we may visit in a most picturesque region the beautifully preserved craters of extinct volcanoes in the Auvergne of Central France ([105], [124], [145]), which district is entered from Clermont-Ferrand. Here are found the characteristic puys, steep lava domes of viscous lava ([105]), which figured largely in the early controversies of geologists concerning the origin of rocks.

Fig. 493.—Outline map of a geological pilgrimage across the continent of Europe.

The rest of our pilgrimage will be so planned as to enter the noble river Rhine at its mouth ([Fig. 493]), ascend its course to its birthplace in the snows of Switzerland, and after further exploration of the features of this fretted upland, traverse northern and central Italy so as to make our departure for America by the southern route. Entering then upon this course in the Low Countries, we have first the opportunity of observing the characteristics of a great delta with natural levees artificially strengthened as dikes ([165-168]). Here also are found dunes of beach material which has been raised by the wind into a great rampart near the shore ([209-211]). Such a wall of dune sand is well displayed at the bathing resort at Scheveningen near the Hague ([421]). The flood plain of the Rhine ([162-165]) may be studied in a journey up the river to the university town of Bonn, from whence a day’s excursion should be devoted to the relics of volcanoes known as the Seven Mountains (H. von Dechen, Geognostischer Führer in das Siebengebirge, Bonn, 1861). As a preparation for this trip and others in the volcanic Eifel higher up the river, a visit should be made to the mineral and rock collections of the Poppelsdorfer Schloss at the University. In the volcanic Eifel are found some of the most interesting of crater lakes ([405]), the largest being Lake Laach with its somewhat peculiar volcanic ejectamenta and its picturesque abbey (see von Dechen, Geognostischer Führer zu der Vulkanreihe der Vorder-Eifel, etc., Bonn, 1886. Consult also Lane, A Geological Tourist in Europe, l.c.).

Continuing our course up the river from Bonn, we soon enter the gorge of the Rhine cut in an uplifted peneplain ([169], [171], [174]). From Coblenz, where the Moselle enters the Rhine, a side trip may be made up this tributary river past Zell with its entrenched meanders ([173]) to the ancient Roman city of Treves. Above Bingen on the Rhine we leave behind us the narrow gorge and rapid current of the river and continue over the broad floor at the bottom of a rift valley ([403]), lying between the forest of Odin and the Black Forest on the east and the “Blue Alsatian Mountains” far away to the west. At the margins of this plain are beds of loess with their characteristic joint structures and inclusions ([207]), and in the higher hills on either hand a wealth of intrusive igneous rocks.