Formations that look like fossils
Our collections include some weird and interesting rocks, including some that look just like fossils, known as pseudo-fossils. But fossils are the remains of living plants and animals (organic), and most rocks are inorganic. Here are some of the fascinating and eye-catching rock samples from our museums.
Fossil ripple marks and mudcracks
Ripple marks (shown right) were formed by the action of waves in shallow water. At some point in time, these ripples were covered by sediment, preserving them perfectly.
Mudcracks formed from sediment deposited in water. When the water drained away, the sediment (mud) was baked hard and cracks appeared. The next influx of water carried more sediment which was dumped on the surface, filling the cracks and preserving them.
These features are really helpful in determining past environments. They show that the rocks must have formed in lakes or lagoons millions of years ago.
Crystals and gems
The formation of crystals is a slow geological process, which takes place where mineral-rich liquid fills cracks and voids in rock. Calcite is the most common crystal on earth and comes in many shapes and colours. The stunning Portland Gem (right) consists of rosette-shaped calcite crystals, radiating from small stalagmites on a limestone slab.
Dreikanter – sand-blasted pebbles
In areas where the wind usually blows in one direction, a rock face can become flattened and polished by sand and debris. This forms wind-shaped, sand-blasted pebbles called dreikanters (shown left). The bottom side of the pebble is flat, but the sand has shaped everything above it into a triangular shape (the word dreikanter is German for ‘three-edged’). Dreikanters generally form in dry, arid environments from hard rocks.
Flint formed from silica that dissolved away from the remains of sea sponges buried in the chalk. The silica then reformed in cavities in the chalk, most commonly burrows of marine worms and shrimps. This is why many flints are tube-shaped, branching and hollow.