Notes on the formation of extensive conglomerate and gravel deposits

Abstract

This paper (extracted from the Philosophical Magazine of March, 1830) investigates the adequacy of currently observable geological processes ("actual causes")—specifically the action of seas and rivers—to explain the formation of extensive, ancient conglomerate and gravel deposits found in the Earth's crust.

De la Beche examines three main mechanisms:

1. Action of Tidal Seas on Coasts: The sea primarily acts to throw rounded gravel (shingles) back onto the land, forming beaches and barriers (such as the Chesil Bank), especially during gales. There is little evidence of these large pebbles travelling outwards into deep oceanic waters to be consolidated.

2. Action of Rivers on their Beds: Rivers are most effective at producing gravel in rapid, mountainous regions. However, natural obstacles, such as lakes, frequently interrupt their course, causing them to deposit their detritus locally (e.g., the Rhone and Ticino depositing gravel in the Lakes of Geneva and Maggiore). Long rivers typically transport only sand or mud to the sea, not pebbles.

3. Discharge into Seas (Tidal/Tideless): Large rivers discharging into seas generally carry fine detritus (sand/mud) to form deltas or bars. Small tidal rivers are often blocked up by the sea, which forces the detritus back towards the land.

Conclusion: The author argues that contemporary processes, acting over time, can only produce gravel/shingle along narrow coastal lines or limited riverbeds. To account for the extensive, widespread, and voluminous conglomerate deposits observed in the geological record, a greater and more general force is required. This force is attributed to violent movements in the mass of waters—debacles or deluges—which must have resulted from the great, planet-wide convulsions and dislocations of strata (faults and mountain building) that occurred at various epochs. These major catastrophic events would have provided the power needed to collect and amass vast quantities of gravel.