The Somme Terraces

RIVER TERRACES CAN be grouped either according to the height of the erosional surface or bench underlying the deposits or according to the maximum height attained by the fluviatile deposits on it. Both systems have been used in classifYing the Somme Terraces. V. Commont (1910) studied the terraces in the region of Amiens, and defined them on the basis of the height of their benches measured from the floor of the buried channel at that locality:

High or 45-m Terrace Middle or so-m Terrace Low or 10-m Terrace

General de Lamothe (1918) studied them in the light of his experience of sea-levels in the Mediterranean. Plotting the level ofthehighestfluviatiledepositson each terrace, he reconstructed the profile of the river at the end of each of the main aggradations. The surfaces of these sheets ofaggradation have a gradient diminishing seawards and running into the former high sealevels as follows (Zeuner, 1959a, p. 126): Sheet of La Ferme de Grace (at Amiens resting on 45-m bench)

graded to sea-level 57-58 m above OD. Sheet of St Acheul (at Amiens resting on so-m bench) graded

to sea-level S2-SS m above OD. Sheet of Montieres (resting at Amiens on 10-m bench) graded

to sea-level 18-19 m above OD. It is sometimes wrongly assumed that the rock-benches of the

terraces slope seawards parallel to the surfaces of the aggradation sheets, but in fact their gradient is much steeper. The

c

rock-benchesrepresentthelevelstowhichtheSommecut downduringlowsea-levelstages,andthesedidnotlastlong enoughfortherivertobecomegradedtobase-levelby erosion.