2949775586

118

T. Wilgat et al.

Although the amount of circulating water is smali, there are nunerous lakes — natural and changed to reservoirs, smali ponds, wetlands of various types, streams and ditches periodically discharging water. The secret of the apparent abundance of water lies in the relief and geological structure. This

Fig. 1. Lęczna-Włodawa Lakę Region

1 — region boundaries, 2 — lakes, 3 — lakes turned into retention reservoirs, 4 — ponds,

5 — Polesie National Park, 6 — Landscape Parks, 7 — colliery

morphological fiat depression is open eastwards and surrounded by higher terrain on three sides. The altitudes vary from 180—200 m a. s.l. on its Southern and northern peripheries, to 160 m by the river Bug and 140 m in the north—west. Plains inclined below 2° are the main element. But the origin of the monotonous plains differs. Two denudation and two accumulation surfaces can be distinguished here (Wilgat 1957). The accumulation plains, differing slightly in their heights, contrast with their moisture-conditioned vegetation. The lower ones have mirę vegetation, whereas the higher sandy ones are cultivated or used as pasture, or partially covered with forest.

The sandy plain was formed in the period of the Middle-Polish glaciation, by the accumulation of stagnant and flowing waters. In the last glaciation it was overlain with sands. Numerous depressions in its surface were filled with organie sediments in the Holocene, and they now form a large — discontinuous — lower accumulation plain.

The denudation plains, common in the northern part of Polesie, occupy smali areas in the Lakę Region. They were formed on ground moraine or Upper Cretaceous outerops. They are a little higher than the accumulation plains and have a diversified microrelief.