Dr Cornwell is correct in his observation that if all sand and gravel in the Stour tunnel-valley has the same density as the Chalk, the measured Bouguer anomaly in the area should be interpreted only as a varying thickness of boulder clay. This is something the authors were aware of, and it is discussed in page 109 of our paper although obviously not in precise enough terms. A reinterpretation along these lines was not carried out for several reasons:

The range of density determinations (including those presented by Dr Cornwell) for a single formation is of the same order of magnitude as the density contrasts used in the interpretation.

The conclusion that the density contrast between chalk and gravel is very small is derived from a study of the optimum interpretation densities. The errors in these and in the three points used in the plot of our Fig. 7 are almost certainly very large, especially remembering that these values have also been derived through interpretation in terms of total valley fill. So, although a near zero chalk/gravel density contrast is suggested, this is far from certain.

The high sensitivity of any depth calculations to small errors in observation had been demonstrated. This is always true where small observed anomalies and small density contracts are being considered.

It was the aim of the geophysical surveys to map the depth to the Chalk. Our approach was therefore to carry out an interpretation of the gravity survey in those terms using what little

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