Reading the Rock Record

Geologists established the order of the rock record, and much of its relative timing, before any dating method existed. The principles used are observational and were worked out between the seventeenth and nineteenth centuries, largely by people who accepted the biblical chronology.

This matters for the age question. The sequence is not derived from radiometric dating; dating was applied afterwards to a sequence already established, and agreed with it.

The principles#

Steno was a Catholic bishop. Smith was a canal surveyor with no theological programme, and worked out faunal succession because he needed to predict which rocks lay ahead of his excavations.

What each principle establishes#

Cross-cutting relations give relative ages without any assumption about deposition rate. If a granite intrusion cuts through sedimentary layers, the layers existed first. If a fault displaces the intrusion, the fault came later. This produces an order from geometry alone.

Inclusions work the same way. A conglomerate containing rounded pebbles of an older rock requires that the older rock existed, was broken up, and was rounded by transport before the conglomerate formed.

Faunal succession is the principle with the most force, because it is a global correlation. Smith observed that a given assemblage of fossils always occurs in the same position relative to other assemblages, wherever the rocks are found. This allows strata in England to be matched with strata in France without any physical connection between them.

Smith published his geological map of England and Wales in 1815, and it is close enough to modern maps to remain usable.

Unconformities#

An unconformity is a surface where deposition stopped and erosion occurred before deposition resumed. They record elapsed time directly.

Hutton's Unconformity at Siccar Point in Scotland, described in 1788, shows near-vertical greywacke beds overlain by gently dipping red sandstone. Producing it requires: deposition of the lower beds, their tilting to vertical, erosion of the tilted surface, then deposition of the upper beds, then tilting of those.

Each step is a process observable today, and each requires time. Hutton's companion John Playfair wrote that the mind grew giddy looking into the abyss of time, and Hutton concluded there was "no vestige of a beginning" — before any means of assigning numbers existed.

Features requiring elapsed time#

Certain features cannot form during deposition and record pauses.

Common objections#

"The geologic column is dated by fossils and fossils are dated by the column"

The standard circularity objection, made by Henry Morris and repeated widely.

The order was established before any dating method existed and before evolution was proposed. Smith's succession is an empirical observation about which assemblages occur above which, and it can be verified by anyone examining a cliff face.

Absolute ages come from radiometric dating of igneous rocks — volcanic ash beds and intrusions — which is independent of fossils entirely. The two are then cross-checked, and the sequence radiometric dating produces matches the sequence Smith established a century earlier.

The circularity would be real if the only evidence for a layer's age were the fossils in it and the only evidence for the fossils' age were the layer. Neither holds.

"Layers can form rapidly, as at Mount St. Helens"

Correct, and the 1980 eruption is a genuine demonstration. It deposited about 7.6 metres of finely laminated sediment in a few hours, and subsequent flows carved a canyon through it.

The observation is accepted in geology, and rapid deposition is not disputed — turbidites and flood deposits are well studied.

Rate of deposition is not the point at issue. The question is whether the whole record formed rapidly, and the features above answer it: a buried soil with root traces, a desiccation-cracked surface, and a burrowed hardground cannot form during rapid deposition, because each requires deposition to have stopped.

The Mount St. Helens deposit contains none of them, which is what a genuinely rapid deposit looks like.

"Strata are out of order in places, so superposition fails"

Older rock does sit above younger rock in some locations. The Lewis Overthrust in Montana and the Glarus thrust in Switzerland are the standard examples, and both are cited by young-earth authors.

These are identified as thrust faults on physical evidence, not assumed. The contact surface shows fault breccia — crushed and pulverised rock — along with slickensides, polished grooves produced by rock sliding against rock, and mylonite where the rock has been deformed under pressure.

If the sequence were original, the boundary would be a normal depositional contact, and it is not. The fault plane at Lewis is traceable for hundreds of kilometres and the displacement is measurable.

"Unconformities could be erosion surfaces from within the flood year"

Flood models do invoke internal erosion surfaces, and erosion certainly occurs during any large depositional event.

The difficulty at Siccar Point is the tilting. The lower beds were deposited horizontally, lithified, tilted to vertical, and eroded flat before the upper beds arrived. Tilting requires tectonic deformation, and lithification requires cementation.

Some unconformities also carry palaeosols with in-place root systems and soil horizons, and karst surfaces with dissolution features. These form on land, over long periods, and cannot form underwater during a flood.

What the evidence shows#

The order of the rock record was established by observation between 1669 and 1816, before radiometric dating existed, before evolution was proposed, and largely by people who held to a biblical chronology.

The principles used are geometric and observational: what cuts what, what contains fragments of what, and which fossil assemblages occur above which.

Radiometric dating was applied later to a sequence already fixed, and agreed with it. Related material is in The Geologic Column and Time in Sedimentary Rock.