Page 1 Geology and soils

Geology and soils

To understand the vegetation (and how it changes),  you need to understand the soil (and how it changes).  To understand the soil, you need to understand the geology and landscape (time, weathering, slope, drainage, etc).

Geology maps are available from BGS:

                            Key:

                Pink – Kesgrave sands and gravel 

                Red – red crag – coarse sand and shells, iron-stained, phosphate pebbles (Coprolite)

                Brown (Tms) – clay (septarian concretions in clay = cement, excavated at Harwich). 

                Beige – river terrace deposits (u T), sand and gravel

                Yellow – alluvium, mud

You can see how the geology underlies the landscape using the BGS viewer.  You can change settings, basemap, etc using the menu.

Understanding the geology

250 million years ago (mya), 'England' was within Pangea, a continent at the equator.  Pangea started splitting up 200 mya and our part started moving northwards, but covered by sea.

Chalk, underlying all our soils, was deposited 70 to 100 million years ago in a tropical sea.  London clay was then deposited on top of the chalk, 50 mya (still in a tropical sea).  The layer of Red Crag is 1.5 to 3.75 million years old (deposited in a tidal marine bay), and at its base it often contains Coprolite nodules, rich in phosphate, which led to them being extracted from the Crag to make fertiliser in recent times.

The Ice Ages began 2.4 million years ago and lasted until 11,500 years ago. There were at least 17 cycles of glacial and interglacial periods. The glacial periods lasted longer than the interglacial periods. The last glacial period began about 100,000 years ago and lasted until 25,000 years ago. Today we are in a warm interglacial period.

The great ice-sheet just reached the area, diverting the course of an earlier river which had deposited the sands and gravels that make up so much of the soil along the coast. It is these infertile soils and the free-draining nature of these sands and the underlying Crag that has allowed the characteristic dry heath landscape of coastal Suffolk to become established.

Rivers carved the valleys of the Deben and Fynn, exposing the layers of ancient sediments.

A technical review: Geology of the Woodbridge and Felixstowe district

2 million years on the Suffolk Coast  by Tim Holt-Wilson

GeoSuffolk   GeoSuffolk website 

Crag contains coarse sand and shells.  The shells, which are made of calcium carbonate, were applied to acid soils, as found locally, to raise the pH and make the soil more productive.  In some layers at the base of the crag, coprolite is found, which can be used as phosphate fertiliser. Crag and coprolite. Over the last century, artificial fertilisers and lime became widely used and were much more effective than crag.

 Photo: crag with shells (as seen at Levington)

Suffolk archaeological service:  suffolk historic environment record

Soils at Martlesham Wilds

The soils reflect the underlying geology.  they mostly consist of loamy sands but there are sandy clay loams in the valleys and clays near the river Deben and Martlesham Creek.

The sandier soils tend to be acid where they have not been under cultivation e.g. Lumber Wood.  The soils that were intensively cropped (usually they have a pH around 6.5 for healthy crop growth) will slowly become more acidic.

Data will be gathered from research and surveys.

Soil Walks

Walks that inform visitors about the soils present at the Wilds and organised every year.  The first one was in April 2024 and looked at the soils near the car park and then near Lumber Wood in the small grassy field.

A second walk was on 1st May 2025 and looked at soils on the edge of Lumber Wood, alongside the path of the grassy field and near the site of the 'Ghost Pond'.

The soils are generally acid (around pH 5) in the sandy soils on the margins of the fields and the woods, and nearer pH 6 in the old arable fields.