A walk through Old Red Sandstone
This geotour, which will take you around 45 minutes to an hour, starts at the large car park NO237488 and then follows the path signposted down to the Den of Alyth/Playpark.
Details of what can be viewed along the route can be found below.
On your left are the cliffs of the old Alyth sandstone quarries. The sandstone is dull reddish brown, fine to medium grained and plane bedded, flaggy in places, and was an excellent building stone.
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The layers or beds dip southward at about 10 degrees. These sandstones were deposited in a relatively low-energy environment.
The two boulders in the play area are glacial erratics: granite and hornblende schist.
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Now walk back upstream, noting that the same sandstones form the river cliffs opposite. Beyond a slight rise in the path at NO23474867 a band of dark grey rock crosses the burn and both sides of the Den have been quarried away. This is a dyke of basalt (locally known as whinstone). It was intruded into the sandstone as a near-vertical sheet.
Columnar jointing can be clearly seen when the river level is low.
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The photo below, shows the quarry on the far (south) bank. The quarried stone does not appear to have been used for building in Alyth, so probably went for aggregate.
At the footbridge NO23354876 there are some more granite and schist boulders in the burn. 20m farther on by some railings there is an outcrop of sandstone with pebbly sandstone lenses.
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The pebbles are of volcanic rocks and vein quartz with very few schists. They are rounded and mostly small (<5cm). The poorly sorted beds of pebbly sandstone were deposited in high-energy flood events.Continuing upstream, the rocks in the bed of the burn and the cliffs opposite dip gently SSE ; older rocks come to the surface father west. Cliff exposure stops at the next footbridge and just before it is another (very mossy) granite erratic at NO23084878.
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Return along the den or take one of the higher paths back to the car park.
