I think it's this,
Pecking and rubbing:
A design can require anywhere from five hundred to a thousand blows. The design usually starts with a series of small dots which are then connected together by continued pecking. Flat areas can be made by continued pecking or by rubbing the surface smooth with a flat stone.
http://www.ancestral.com/materials.html

The figures were executed by two methods. In one the figure was outlined
by grooves pecked and rubbed into the sandstone. The other method was by rubbing the surface so that the entire form of the figure was depressed.
http://www.wvculture.org/shpo/nr/pdf/harrison/76001937.pdf

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Note added at 2 hrs (2017-01-02 18:33:35 GMT)
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More examples:

" The Libyco-Berber script and the Canary Islands
7.1 The archaic script
The above mentioned original type of alphabet, which can be called
the archaic one, spread to the East as far as Kabylia and to the West
to the Canary Islands. The most plausible time of this taking over o the islands deduced from the epigraphic development is the 6th
century BC (Fig. 3).
We can find these archaic inscriptions especially on El Hierro and
Gran Canaria and single ones on La Gomera and La Palma. Their main
features are (Fig. 4):
1. they are pecked (and not carved or scratched)
2. they prefer round variants (instead of angular ones)
3. they occur in a clearly definable context of linear and
geometric depictions: circles, serpentines, labyrinths, nets etc."
https://www.researchgate.net/publication/259564172_The_colon...

"Petroglyphs are designs engraved on rocks by carving, pecking, rubbing, or a combination of these methods."
http://www.jstor.org/stable/j.ctt130hv8b