Just below the topsoil layer is the subsoil layer. The subsoil may contain some broken down organic matter but it is mostly made of weathered rocks and clay minerals.

‘Subsoil’ refers to the stratum of soil immediately below the surface soil or topsoil. Often this layer is overlooked, as most land management is focused on the topsoil, which can be altered drastically by tillage and other practices.

Subsoil is an important storehouse of moisture, especially since it’s usually much thicker than the topsoil, and the moisture isn’t lost as easily by evaporation. The higher clay content of subsoils makes for higher water holding capacity, too.

Pore water pressure (sometimes abbreviated to pwp) refers to the pressure of groundwater held within a soil or rock, in gaps between particles (pores).

Subsoil colors are classified as bright, dull, or mottle colored. Bright-colored subsoil is characteristically brown, reddish brown, or yellowish brown. Good drainage is what gives subsoil a bright color. This is because the iron found in the soil has been oxidized.

Bedrock is the hard, solid rock beneath surface materials such as soil and gravel. Bedrock also underlies sand and other sediments on the ocean floor. Overlying material is often unconsolidated rock, which is made up of loose particles.

7 billion-year-old stardust is the oldest stuff on Earth. Microscopic grains of dead stars are the oldest known material on the planet — older than the moon, Earth and the solar system itself.

The zircon crystals from Australia’s Jack Hills are believed to be the oldest thing ever discovered on Earth. Researchers have dated the crystals to about 4.375 billion years ago, just 165 million years after the Earth formed. The zircons provide insight into what the early conditions on Earth were like.

zircons

Feldspar

4.375 billion years old

The Earth is some 4.6 billion years old. The oldest known rocks (at 3.9 billion years) occur within the Archaean – a system which extends to 2.5 billion years. So Archaean rocks can presently be found over an age range of 1.4 billion years.

4.54 billion years

4.543 billion years

The principle of superposition states that the oldest sedimentary rock units are at the bottom, and the youngest are at the top. Based on this, layer C is oldest, followed by B and A.

To establish the age of a rock or a fossil, researchers use some type of clock to determine the date it was formed. Geologists commonly use radiometric dating methods, based on the natural radioactive decay of certain elements such as potassium and carbon, as reliable clocks to date ancient events.

The location of fossils in rock layers provides evidence of Earth’s past landscapes. It is hard to guess the age of rock. Scientists have to act like detectives, piecing together a mystery to determine how long ago rocks formed. Fossils found in a particular rock layer help scientists determine the age of the rock.

A fault is a break in Earth’s crust. A fault is always younger than the rock it cuts through. The surface where new rock layers meet a much older rock surface beneath them is called an unconformity. An unconformity is a gap in the geologic record.

Magma that cools and pushes into. bodies of rock and hardens is called an. Intrusion. An intrusion is always younger than the rock layers around and beneath it. The intrusion (in red) is now younger than the surrounding rocks.

fault

An intrusion is a body of igneous (created under intense heat) rock that has crystallized from molten magma. Gravity influences the placement of igneous rocks because it acts on the density differences between the magma and the surrounding wall rocks (country or local rocks).