Pleistocene Geology of Moravia Quadrangle 
353 
(2) It appears furthermore that the course taken by a loop in 
crossing a valley depends also on the depth of the valley. In deep 
valleys which have gently sloping side walls, the tongue of ice 
reaches farthest ahead of the main ice-front. Consequently the 
loop formed is more symmetrical, the lateral segments being 
many times longer than the flood plain segment. The lateral 
segments likewise drop so gently into the lowest part of the valley 
that they present a diagrammatic^ appearance. It is evident, 
therefore, that this control is better illustrated in mature valleys, 
such as those now occupied by the Finger lakes. 
(3) The form of loop may also reflect a lack of symmetry in the 
cross-section of the valley. It not infrequently happens that a 
spur of one side-wall is opposed to a gentle slope on 'the opposite 
side. A cross-section of the valley at this point gives un symmetri- 
cal slopes, and exerts a control on a loop developed there. Such 
a control gives the drift-loop on one of its sides a very straight or 
ridge-like appearance, the direction of the ridge marking the axis 
of feeding of the ice-tongue. 
(4) The relation sustained by the volley axis to the direction of 
ice-movement will also have an influence on the general outline of 
valley loops. Usually the topography exerts a strong control 
over the direction of ice motion along its more attenuated front, 
but this control is effective up to a certain relation of these axes, 
beyond which the direction of motion of the ice sheet decides the 
course taken by the loop in crossing a valley. Thus it happens 
that a loop, near the point of junction of two or more fairly mature 
valleys, may sustain a position anywhere between coincidence with 
the axis of the valley and at right angles to this axis. 
(5) The influence of marginal streams, as described by Tarr,* 
is frequently shown in the lateral segments of loops, producing 
sometimes a lopsided developm.ent. Slight oscillations of the 
ice-front cause a shifting of streams lateral to the valley lobes; 
both the erosion work of these streams and their unequal deposi- 
tion of load tend to the asymmetrical development of loops. 
^ R. S. Tarr, Bull. Geol. Soc. Am.^ vol. 16 (1905), p. 218. 
® Bull. Geol. Soc. Am.^ vol. 16 (1905), p. 222. 
