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
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