Geology of Christchurch Park

Field Excursion Report: a guided walk to examine the geology of Christchurch Park, 21 July 2011. Leader Bob Markham

Bob always draws a good crowd for his excursions and this was no exception with 33 people attending on an overcast but warm summer evening. The walk covered the southern part of the park.

We started at Christchurch Mansion, made largely of bricks from the local Eocene London Clay. Such an early use for bricks (1594) was an indication of the great wealth of the Mansion owner. Incorporated into the structure as both structural and decorative elements are blocks of pale, creamy Caen Limestone. The approach to the Mansion is a path of Yorkshire flags, slabs of Carboniferous Sandstone, ending at the front doorstep with a slab of Jurassic Purbeck Marble, in which on close inspection could be seen a mass of fossil snail shells. The formal front garden, completed in 1932 is also laid out with cobbled flints from the Bullhead Beds, again local and Eocene in age, and with a row of crystalline granite bollards, certainly not a local rock!

Close to the western side of the building was a Sarsen stone, more of which were seen later in the walk, on a bed of grit made from ground up granite. At the rear of the Mansion, in the Wolsey Garden also created in 1932 when the Art Gallery was built, was another

Sarsen with a sculpture by Bernard Reynolds. Bob then showed us a decorative Caucasian Wing Nut tree (Pterocarya fraxinifolia) of geological interest because the species existed in England long before the Ice Age, some 2.5 million years ago.

We then walked across to the Round Pond, originally created as a fish pond by the Augustinian monks, fed by natural springs. Indeed, springs became a feature of the walk: rain soaks into the permeable sands and gravels of the Kesgrave Formation (deposited before the Ice Age about 750,000 years ago by the forerunner of today's Thames) and issues forth where it overlies the London Clay. The Kesgrave Formation contains pebbles of quartz and quartzite, seen in a small outcrop in the cutting through the bird reserve. Many springs, oozes and seepages were seen during the walk. The springs at one time fed into the brook from which Upper and Lower Brook Street get their names.

In the area of The Wilderness Bob and Caroline demonstrated the evidence for slope processes, where sands and gravels slip or slump over the London Clay, lubricated by the springs. Backward leaning trees in the cutting indicate rotational landslip movements, while forward leaning trees near the Wilderness Pond indicate soil flow and slumping.

We paused at the rockery in the Lower Arboretum. Here Bob explained the origin of the Sarsen stones, from which the rockery is constructed. The stones are a hard sandstone (the Reading Beds) which underlie the London Clay. Geologically referred to as silcretes, the one-time sands were lithified by minerals (particularly silica) deposited by illuviating and evaporating ground water, the product of a once much warmer climate. Many of the Sarsens showed a well mammilated (bumpy) surface texture. The Sarsens were discovered during the excavations for the Wet Dock in the l840s, before being hauled to their present resting place.

We paused again by the upper gate to Fonnereau Road in Mayors' Walk to see the plaque to Sir Edward Packard next to a tree planted by him. Packard was a leading manufacturer of artificial fertiliser made from phosphatic material excavated from the Suffolk Crag deposits during the 19th century - the locally important 'coprolite' industry (hence Coprolite Street on the Waterfront). Packard helped create the first museum built in Ipswich and was chairman of the committee for some years. He also has a fossil Red Crag beaked whale (Choneziphius packardi) named after him.

Nearing the end of the walk, we stopped at the First World War Memorial, made of shelly Oolitic Portland Stone, which was deposited in a shallow tropical shelf sea some 150 million years ago during the Jurassic. A few metres away is the memorial to the South African War, made of Darley Dale Sandstone from the Carboniferous Millstone Grit of Yorkshire, deposited in huge river deltas some 300 million years ago. The memorial dating from 1906 was moved here from the Cornhill when Lloyds Avenue was built. We ended the walk on the southern side of the Round Pond, on the dam built to retain the water, a reminder that the ponds are man- made. Throughout the walk Bob had referred to the role the Ipswich Society had played in developing the assets of the park. And Bob had kept us enthralled and entertained for a fascinating 1.5 hours; he had given us a glimpse into the relevance of earth science and just how it affects not only our landforms but also our history and everyday lives.

Roger Dixon