We had a lovely sunny evening for our walk with 30+ Ipswich Society members down the side and across the bottom of the River Gipping/Orwell valley, considering the geology in the buildings and under their foundations.
We started with the Elim Pentecostal Church, with its distinctive Kentish Ragstone, a sandy limestone about 120 million years old, seen in many of the Victorian churches built after the arrival of the railway to our town. Opposite, the old telephone exchange in Portman Road was built in 1954, its foundations showing glacial boulder clay (about 440,000 years old) with gravel above it which yielded a mammoth tooth. Downhill, St Matthew's Church has London Clay septaria in its walls. This local calcareous mudstone, about 54 million years old, is not a good building stone - its history includes church tower collapses, such as at Alderton. The foundations of the underground car park across Civic Drive showed much flexuring of the boulder clay/London Clay layers, buckled up by the advancing ice. The problems this caused during construction may be reflected in the chancel arch of St Matthews, which is slightly askew perhaps due to this unstable foundation.
Further down Portman Road, excavations have shown mud and then peat, with branches and nuts resting on gravel. This gravel is the lowest river terrace deposited by the Gipping during the last cold phase about 25,000 years ago and the peat on top of it represents a younger river channel. We traced this - a flat, grassed linear feature between Canham Street and Little Gipping Street (the clue is in the name), it is occupied by the low level sewer now. It leads to the Alderman Canal, though cut off from it now, and we walked along the canal path discussing its origins and water supply. The canal is banked up 3 metres above the recreation ground - an artificial mill leet (elevated water supply) for the former Handford Mill. Could the canal receive water directly from the Chalk in the Dillwyn Street area, where the gravel is banked up against the Chalk? Our progress to the Handford Sluice revealed the penstock valve which can let water in from the River Gipping. The Handford Sluice is (one of two) where the fresh water Gipping ends and the salt water Orwell begins - holding back the Orwell at high tide.
We walked along Sir Alf Ramsey Way on the lowest river terrace to the Bus Depot, with its diverting evening bus-shunting activities. The Chalk is only a few metres below the surface here and our modern steel-framed buildings all the way along to the waterfront have their foundations driven into it. Excellent - except for a deep, narrow channel eroded under the ice 440,000 years ago which snakes its way from the Great Blakenham area via the bus depot (and the ‘Wine Rack') down to the Orwell bridge. This steep sided channel in the Chalk can be 30-40 m deep - it is infilled with sand and clay and requires deep piles driven down to the Chalk. CM's experience of the Ramsgate earthquake of May 2015 was related at this point - woken at 3am by a loud bang and several seconds of horizontal vibrations. Shockwaves from the faulting under Ramsgate travelled northwards through the Chalk to Ipswich (Wolsey Street in this case) and transmitted themselves through the foundations (in the Chalk) and up steel frame to the 5th floor apartment. Two other members of the party had also been woken up by this experience - we can't usually see the Chalk in Ipswich but sometimes we can feel it!
Still on the 25,000 year-old terrace, we stopped by the old power station in Constantine Road. Various excavations here have yielded mammoth and reindeer bones indicative of cold climate, and also evidence of ourselves - some (very beautiful) Late Palaeolithic flint implements. Did these mammoth-hunters experience earthquakes in the Chalk?
Caroline and Bob Markham