Alexander AGASSIZ - The naturalist in Nicaragua

We started on our return down the valley at three o'clock, and took particular note of the succession of the rocks, as I had become much interested in finding these quartz and gneissoid beds, which I had no doubt were the same Laurentian rocks that I had seen in Canada and Brazil--the very backbone of the continent, ribbing America from Patagonia to the Canadas--the fundamental gneiss which is covered, in other parts of Central America that I had visited, by strata of much more recent origin. Going down the valley of the Depilto the massive beds of quartz and gneiss are soon succeeded by overlying, highly inclined, and contorted schists, and as far as where the road from Ocotal to Totagalpa crosses the river, the exposures of bed rock were invariably these contorted schists, with many small veins of quartz running between the laminae of the rock.

On the banks of the river, from about a mile below Depilto, unstratified beds of gravel are exposed in numerous natural sections. These beds deepen as the river is descended, until at Ocotal they reach a thickness of between two and three hundred feet, and the undulating plain on which Ocotal is built is seen in sections near the river to be composed entirely of them. These unstratified deposits consist mostly of quartz sand with numerous angular and subangular blocks of quartz and talcose schist. Many of the boulders are very large, and in some parts great numbers have been accumulated in the bed of the river by the washing away of the smaller stones and sand. Some of these huge boulders were fifteen feet across, the largest of them lying in the bed of the river two miles below Depilto. Most of them were of the Depilto quartz rock and gneiss, and I saw many in the unstratified gravel near Ocotal fully eight miles from their parent rock. Near Ocotal this unstratified formation is nearly level, excepting where worn into deep gulches by the existing streams. The river has cut through it to a depth of over two hundred feet, and there are high precipices of it on both sides, similar to those near streams in the North of England that cut through thick beds of boulder clay.

(PLATE 22. GEOLOGICAL SECTION NEAR OCOTAL. Section of Strata between Depilto and the hill three miles south-west of Ocotal.

Gravel with boulders of trap and conglomerate.

Gravel with boulders of gneiss and quartz rock.

Contorted schists.

Quartz rock and gneiss.)

The evidences of glacial action between Depilto and Ocotal were, with one exception, as clear as in any Welsh or Highland valley.

There were the same rounded and smoothed rock surfaces, the same moraine-like accumulations of unstratified sand and gravel, the same transported boulders that could be traced to their parent rocks several miles distant. The single exception was, I am convinced, one of observation and not one of fact, namely, I saw no glacial scratches on the rocks; but geologists know how rare these are on natural exposures in some districts that have certainly been glaciated, and will not be surprised that in a hurried visit of only a few hours I should not have discovered any. Glacial scratches are seldom preserved on rock surfaces exposed to the action of the elements. Even in Nova Scotia, where scratches and grooves are met with wherever the rock surface has been recently laid bare, I do not remember having ever seen any on natural exposures. It is only where protected by a covering of clay or gravel from the action of the elements, that they have been preserved through the ages that have passed since the glacial epoch, and as I did not see any rock surfaces near Depilto that had